I am curious to how useful they actually are in this game. I'm in the 2nd round of the draft and there is one I have rated as a 94, but I could go with a 77 WR with good speed and I think the WR is probably still the better pick.

I love them but I would pick a Wr over a fullback all day! If I had 2 Wr already then I would take the FB

Tall fullbacks are sometimes a steal if they can make transfer to TE. Also fast FB's can sometimes be good RB candidates.

As far as switching positions goes, how much does a players weight really matter? If I have a guy with the attributes to be an incredible guard, but he weighs 240, will he be terrible at guard because he is way too small, or does the game not take that into account?

It does take it into account, however I can't attest to what extent.

That said, if you are patient they will slowly gain/lose weight towards the ideal for their switched position. So if you're willing to wait three to four seasons that 240 LB player might be hefty enough to make the transition if you switch him to guard now, haha.

I have to say I have trouble with putting a lot into a FB unless he has use as a HB or TE also. No matter their run blocking skill they never pick up the gap guy on lead blocks even if it is in the path of the run, he doesnt seem to start looking for blocks until he clears the line of scrimmage. He is one of the lowest priorities on my teams unless he can do something else.