Bigger gives you perhaps a better chance. The problems with TV signals and any signals really being deflected by trees is that you the antenna can have a lot of gain, but if the signal is being swiss cheesed enough by the trees, no antenna can overcome that.
If I lived at your location, I'd probably investigate a tree mounting. It's a PITA, but if getting the antenna to where it has line of sight conditions to the transmitters, that'll give your antenna the highest chance to provide reliable reception.
Ronard example:
http://www.solidsignal.com/pview.asp...ce=google_base
Any other situation where even if you apply a high gain system on your roof, you still have the tree problem and the antenna(s) can't work with a signal that has been carved up.
Cheers.