Everyone has different preferences on the proportion of the side compared to the middle stone. I don't know any thing about princess or radiant cuts, but for round some say the side stones should be 30-40% of the center, others say 40-50%, and even others say <30% so as to not overshadow the center.
Check this DeBeers ring designer tool out to see how the proportions look:
http://www.adiamondisforever.com/dyoer/index.html
It doesn't hurt to ask several jewelers on what they think. Good luck on your search.
squire
BTW, as you can see from my avatar, I went with the 50% look cuz my FI wanted the DeBeers three stone look.