Well 20 3/4" is just about standard for cabinets fitting 19" monitors. Depending on how bulky your TV is, it ought to fit in that space constraint.
If you're using a smaller monitor like that, though, I'd recommend not making the cab too wide. It'll only make your monitor look smaller. There are plenty of sets of plans online, though, and you can use existing machines as a basis for comparison. One of them is bound to suit your needs.