Since you are new, this is definitely not a naive question. Welcome to greenland and arctic kayaks (the baidarka is from Alaska). Most are made to roll so they tend to be narrow. 21-19 inches seems to be normal. Commercial kayaks, however, seem to be 23-21 for normal yaks. As pointed out already, check out the manufactures webpages to find out what is normal for commercial yaks. Not to say greenland and arctic yaks are bad, but they definitely feel different. There is a learning curve with them. I have a 23 inch wide SOF and a 20 inch wide SOF. They both stay upright, but the wider one feels more stable. It all depends on what you want out of your boat. Do you want a sexy sports car or an SUV?
You have found Tom Yosts website, which is very heavily used and a very good source of boats. If you want a wider boat, then you need to choose a new design. Traditional ribbed yaks are a little more fluid in building and you simply choose the width you want. Traditional ribbed yaks are also much harder to build. Check out www.qajaqusa.org for info
Scott