Generally, remember, generally, what is a good font style and size that most people will be able to read easily please?
Generally… 1em/1.4 sans-serif (“good” depends on individual taste)
(Personally, I will argue that the unfortunately-named comic-sans is one of the most readable and better engineered fonts ever created. Fortunately, there are web alternatives that are acceptably named.)
Thanks ronpat. What size in px ?
You should really not be using font sizes in px
Yep, fine, what size for most people to find easy to read?
See @ronpat’s earlier answer
Aaah, okay, I misunderstood. ;-( I’ve been using px’s for the last 15 years and it’s a hard habit to get out of? How come px’s are not so good nowadays?
Briefly because px don’t always scale for visually-impaired visitors.
I’m sure there’s good explanation on SitePoint somewhere if only I could find it.
Because px cannot be resized by the user. em, rem, and percent can be resized by the user to suit their specific needs, if desired.
If you are accustomed to using px, then you might find rem to be the easiest to adjust to.
rem is based on the font size assigned to the root level of the web page, the <html> level.
html {
font-size:1em;
}
That value will be the basis of other values of rem units on the page.
Whereas em, like percent, is relative to its parent and can be confusing at first.
More about units of measure and font-size:
and
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Times new roman, size 12, is the standard for most legal documents, and most people read it easily. .
But this is the web, not a piece of paper 
Sizes are a completely (and far more complicated) topic on the web.
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Not as easily as you might think.
Following @molona’s lead, the points of serifed fonts tend to cloud legibility on a comuter screen plus the letters in Times New Roman in particular are rather closely spaced which makes them even more difficult to read on computer screen unless the size is increased.
Computer screens and paper are different media and need to be treated as such.
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Go easy on me, and please hear me out, but if someones eyesight is not particurarly good, they would wear glasses, and so wouldn’t for example, Verdana 11px be good for most people to view on any device?
Absoulutely not - the problem with pixels is that unlike all the other measurements available there is no clear definition of how big a pixel is. That text will be readable if the screen is configured to display 5 pixels per inch but will be microscopic if it is set to display 500 pixels per inch.
The only use for pixels is to set 1px for minimum width borders - if you are placing any number other than 1 in front of px you are doing it wrong.
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Glasses cannot correct all forms of visual impairment.
Personally, I would give that an “absolutely not”. Can you imagine the difference in size that an 11px font might have on an 80dpi screen vs a 280dpi screen? Barely legible vs almost microscopic.
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@dez I insist on what’s been said before. You can’t think of fixed measurements when it comes to text on the web. And px are considered fixed because… well, 1px is always 1px. In this case, the difference in size does not depend on the unit but on the monitor itself.
100% is a fairly good reading size and will be whatever the user has decided is best for them. If the user has set no preferences then 100% is likely to equate to 16px in most browsers.
On mobile browsers text under 16px becomes very hard for me to read (even with my glasses on).
This small w3c article is old but still has the same valid points and worth a read,
Thanks for all the input, it’s appreciated and great that we can easily have friendly discussions here without anyone getting heated 
So, you set the em for the whole website, then set individual text parts as a percentage of that? This means that the viewer can adjust the size they see, themselves?
Have you tried that to see how it looks on various devices?
The forum text is set to 14px (bad Discourse); I changed that to 11px using Firebug.
Admittedly, that’s Arial, and Verdana is slightly easier to read, but this is on my “normal” monitor; on my HD laptop, I wouldn’t stand a chance. (I do wear glasses, and my vision with them is good.) Small amounts of text that size (unimportant links in the footer, for example) are OK for me, but if I arrived at a site where the body text looked like that, I would either give up and go elsewhere, or zoom the text (so you need to ensure that won’t break your pixel-reliant layout).
Now imagine you have cataracts, or some other common condition which impairs vision and can’t be remedied by glasses. Are you starting to understand the problems?
Yes, as @PaulOB said, set {font-size:100%} in the <html> element, then you can use rem units throughout the page and the user can adjust the size they see for themselves. Or, if you prefer, you can assign {font-size:100%} in the <body> element and use em units throughout the page and the user can adjust the size they see for themselves. The difference between using em units and rem units is that rem units are relative to the font size in the foot element, whereas em units behave like percents and are relative to their parent. Ems, like percents, take a little getting used to. As far as the user is concerned, there is no difference.