"From my perspective, I think he's going to be challenged in the NFL," the Redskins legend tells TMZ Sports ... "I think he should've taken baseball. That would have been my suggestion to him."

Of course, Kyler's height has been a HUGE talking point ever since the Heisman Trophy winner decided to forgo a baseball career with the Oakland A's to enter this month's NFL draft.

The Oklahoma QB measured in at 5-10, 207 pounds at February's combine ... and count Theismann in as one of the doubters who says that's way too small for an NFL QB.

And why? Joe tells us he's worried Kyler won't be able to throw over taller defensive linemen ... saying, "It's the mechanics of playing the quarterback position."

Theismann also says he's concerned Kyler won't have as good of an O-line as he had at OU ... and thinks that could have devastating results on his style of play.

But, don't get it twisted ... Theismann tells us he's actually rooting for the QB to succeed at the next level -- and thinks he's "a wonderful athlete."