It depends on your personal preferences, fo course. I think that best choise is to have 2 monitors actually...24" would be eoung in that case, or 27 if it will suit to the table, because they are pretty big haha. Another question is the screen resolution. 27" monitor wiht 1920x1080 and 2560x1440 would be different. I have now 2 monitors - 24" and 27", both 1920x1080 and it pretty convinient. I don't know how do pros play, what size of windows do they use...But on 2 screens you can open 8 tables and see them all at a one time.
Also as an option you can try ultra-wide screen, 29-32" and probably you will open 6 tables on one screen.