A 433 MHz RF module is pretty small; they approach the small size of car key fobs. They're also cheap and used a lot, so lots of programming examples and libraries available.
How big can the receiver be? 50m is a long way for most miniature transmitters (especially inside), but putting a big antenna on the receiver can extend the range significantly.
IF you REALLY want to know, you can compute all this, beginning with the noise figure in dB of the receiver. Add the gain/loss of the receive antenna. Then you can compute the path loss for a 50m path to the transmitter. Then you can begin to compute the ERP or effective radiate power necessary to provide the receiver with it's required signal.
Modify the receiver antenna and the transmitter ERP until you mathematically are able to receive the transmitter's signal.
Bet you don't care that much.
Thank you for your answer westfw. The receiver can be pretty big, like 20 x 25 cm, even with a large antenna. But the transmitter has to be small. I will tell you what it is for:
It is for a singer that has to change her costumes during a show, so she can signal to me that she is ready so I can start the music because a mobile phone would be too large to fit into a pocket inside the costume.
Sometimes these performance halls are larger and the distance to the changing cabin is big, about 30-50m inside a building.