There is a lot of browser extensions to simulate a mobile browser screen size and to switch between them, very useful for testing responsive websites.
Here is a list of some smartphone screen size :
High-end new android smartphones (Galaxy S3, Galaxy Note 2, etc.) : HD (1280x720)
iPhone 5 : 1136x640
iPhone 4 : 960x640
Most android smartphone and almost all Windows Phone : WVGA (Wide VGA : 800x480)
Low-end or old smartphones (including iPhone 3GS) : HVGA (VGA/2 : 480x320)
Old low-end smartphones : QVGA (VGA/4 : 320x240)
So the usual (high-end ?) smartphone width is 800px and I don't know of any smartphone with a resolution lower than QVGA, so basically the minimum browser width in landscape would be ~320px, which is very small and not suitable for browsing without zooming.
There's not a lot of smartphone using resolutions between WVGA and HVGA, so i think the strategy is either you support a width >800px or >480px (width a safety margin).
If you manage to get everything to fit in something like 450px in the smallest responsive mode, you'll be more than fine.