Personal guideline ko sa maximum size: As long as I can carry or move it alone, then it's fine.
which means the biggest I can get is a 50V.
Big is really nice - as long as the budget permits.
32V can be pretty huge if you sit close enough - and since you're upgrading from a 21 inch TV, it would feel like a huge leap especially during the 1st few days.
Problema lang na once you get used to 32V, you'd dream of 42V or bigger soon enough. Ganon talaga yon. Tapos you might be tempted to get another panel tech instead of what you got. If you think you can possibly save more to afford a 42V within 1.5 years, I would strongly suggest that you wait.
...and speaking of plasma, the way Panasonic is hurriedly trying to sell out their current models, it appears to me that the next models coming out soon would be significantly better.