If its a 50mm you need to go a couple of mm over, just so it goes on easier, as it will be sealed nicely with a jubilee clip. You will find it harder to get on if its the same size. Would go on with a bit of effort. May as well go few mm over, wont hurt.
It should say when you buy the hose if the measurement is I/D or O/D.
Not the way I would do it.
If you do remove the pipe on the throttle body and measure the diameter of it with a ruler, it is the outside diameter of the throttle body you are after.
Also bear in mind that the "bump" on the end of the inlet, there to help get the necessary tight fit needed and to stop the hose coming off, makes the outside diameter of the inlet slightly larger. So I would remove a 2MM from the overall measurement to get a closer size.
Either that or measure the inside diameter of the hose that was took off. Be careful not to squeeze the hose out of round but.
When buying silicon hose/bends, the size listed is nearly always the inside diameter of the hose, you want this to match the inlet size.