I feel like I'm missing something and can't figure out what size bottom bracket to buy. I'm planning on building a bike and am picking out all the parts before I buy them. I am planning on buying a Poloandbike CMNDR frame and Sugino Pista Crankset. I was looking on the geometries for the frame and couldn't find a description for the size of the bottom bracket I need. Is this something I would figure out in some other way?
More than likely it's a 68mm wide English threaded BB shell on the frame. You may just want to wait until you get the frame to verify that. The Tech Specs on page for the crankset actually do say that it's designed for "103mm bottom bracket" and "JIS Square Taper". Those should be all the measurements you need (assuming the frame is 68mm English): 68/103/JIS.
If you want a matching Sugino BB, here's one: https://www.ebay.com/b/Sugino-Square-Taper-JIS-Bicycle-Bottom-Brackets/177805/bn_5810397
One simple solution: Search your bike manufacturer website!
Another solution: Go to authorised shimano dealers and ask for advice!