You're asking two questions:
1. Is there a standard look one needs to have to be a credible champion?
2. Is there a standard look one needs to have to be WWE Champion?
1 is the matter in debate currently, but 2 is easily answered by looking at WWE's track record. Especially recently. Michaels in 2002 was the last "small" guy to get the belt, and it was a one-month run to capitalize on his return popularity. Everyone else has been tall, jacked to hell, or both. Rey might be pretty ripped, but he's also pretty short, and so it's hard to envision him as champ, as much as many of us would like it.
Here's the thing, though; if they put the belt on him, we'd find out the definitive answer to question 1, which could change the answer for 2. If only.