I have always heard when using a bench ottoman with a sofa take 1/2 the length of the sofa so it the sofa is 88" the ottoman would be 44" long. In your case you have two perpendicular sofas. If you were to follow this reasoning and you wanted one square ottoman the ottoman should be 44" by 44".

I have also heard that many architects and designer employer the "Golden Ratio" to determine the most pleasing size for a rectangle. But quite frankly I am not sure how you would calculate this in your case. Perhaps other readers could answer this question. You could look on Goggle for information about the "Golden Ratio".

What you might want to do is layout various rectangles on the floor and see which one looks best. You could even make a cardboard template.

Keep in mind if you have leather sofas and want a leather ottoman, that a hide of leather is only so large and you may have to have one or two seams in the top. For example it would be nearly impossible to find a clean leather hide that is 56" square and even 52" square, if you wanted a top without a seam. You might find one that is 44" square. Another good option for a large cocktail ottoman would be a tufted ottoman, these are commonly available. After you purchase the ottoman you might even look for a removable tray for the top.

Thanks! we are actually leaning towards a tufted to break up the leather and add another texture. Would love a sunbrella fabric or the like but cant seem to find anything in a large size. I will absolutely google the Golden Ratio - thanks for the tip!