The site will be 2 columns, the right being bigger than the left.
My problem is how big it should be?
I am fully aware of the principle of visual balance and as such I am trying to find the proportions between the 2 columns that will resonate.
Golden rule here will not do it because it results in a very wide left column, although it could be used somehow.
A grid will just help in the alignment but it will not give answers to the issue of the right column width.
Do you have any ideas from where to start in solving this one?
Is there something that I must take into consideration?
It depends on what is going to be in each column. If one of the columns is going to contain fixed width content then you might as well make that column a fixed width. Assuming that is the case for your left column then the right column can fill the remaining space or 35em whichever is smaller.