There really is no such thing as a sweep spot across the board as far as size of homes goes for best flips. It is largely dependent on a number of things like investor experience level, amount of capital the investor has access to, local market conditions, what the competition has for sale, etc.
Back to generally speaking, it is fact that going from a 2 bed to a 3 bed home can dramatically increase value, same goes for going from 1 bath to 2. It is much less of an impact when you go from 3 beds to 4 and even less from 4-5. Same with baths. As the quantity gets higher, the added value goes down. Square footage certainly factors into values of course, especially when one uses an average price per SF comparison of comps.
What is best for you can only be determined by you and your study of your specific market. Don't spend so much time thinking about what other investors do in other areas, look for what is selling in your market and at what price points along with level of finishes. These have a profound impact on what you should be looking for in a flip opportunity.