How small do you need? The Bulgin Bucaneer 400 series are typically 19mm dia x 46mm long:
Smaller than that and you could look at Fischer or Lemo connectors - expensive but very good quality. The Lemo FFA.0E.303 plug measures 11mm dia x 34mm long:
The other waterproof Lemo option is the 0k series.
The Fischer S102-A052 plug measures 9mm dia x 36mm long.
Not sure if they will sell direct to the public - RS components sell some Lemo and Fischer connectors but their website is down at the moment.