You have not given us much information, but since you talk about "room size" I guess you are working with Aria's Convolution Reverb.

Two things have to happen for Convolution to work.

1. The first you have probably already done, that is to turn convolution on (light up the button). I was surprised that you could choose a room without turning the Convolution on, but I guess that is for A/B comparisons. (I would also be sure Ambience is off, at least until you have established the Convolution sound you want.
2. Second, make sure the send dials are set to send sound to the reverb. You can adjust different instruments differently to produce different placement/effects. Of course, you can also use multiple instances of the Aria player with different room settings if you wish for that effect.)

Hope that helps get you started. If I have misinterpreted your question, please send more information (Finale version & computer Operating System are expected as a preliminary) and ask again.