Monitor size is irrelevant. Its all about the resolution baby.
Download something called pixel ruler:
Pixel Ruler Download
Work out the are of 4 tiled Ongame tables and then make sure your monitor has that resolution.
I fully reccomend aria.co.uk. Look for deal of the day and you can get some great bargains. I got the 30" huge res. Dell monitor which fits 9 tables for about £700, standard price seems to be closer to £1100 on the new model.