I can't really remember if it was hard going from a 27" SDTV to a 57" HDTV 3 years ago, but 5 months ago, I went from 57" HDTV to a HD Projector shot on a screen on my wall at 120"(which I also use as my computer monitor as I type this now). At first it's a little harder because your eyes are not used to having to move so much to look at different parts of the screen. With a normal small tv you never really have to move your eyes, which can make it easier. Now, I'm used to it, and I find it every bit as easy as before, except it has more benefits on a big screen.
Like newer games coming out lately, like GRAW for 360 for example, when have huge draw distances in multiplayer games. On a huge screen, if you are playing as a sniper for example, you can look on the complete opposite side of the map without having to look through your scope and see opponents peaking around corners. On a small tv, you can't really do that.
One thing I do know is that going from a big screen back to small screen is equally as hard at first. I sold my 57" tv to my brother in law, and after a month playing at 120", I went to his house and played a game with him back on my old tv, and it was pretty difficult.
I think it doesn't really have anything to do with what size the screen is, it more depends on what size screen you are used to. But it is ALWAYS sweeter playing on a big screen though.
It also depends on how close you sit too.