Fascinating things, aerials.
At 433MHz a whip/monopole is theoretically 173mm long if mounted in the centre of a perfect groundplane.
In reality it's likely to be a tiny bit different. If you've got time it's worth doing range trials for a range of lengths down to about 16cm. (I normally guess about 16.7cm)
If your device has no groundplane, then performance can be a way off. In this case a helical ('rubber duck') can actually be better. (it's less groundplane dependant)
Or you can buy a decent part for under £5.
Try looking in Farnell for Phycomp 433MHz ceramic parts (there is a nice 12mm x 4mm part on 433MHz) or RadioSolutions M4 screw-stud mounted helicals.
Also look at the AF (Antenna factor, or Linx) parts in the Digi-Key catalogue for connector 'rubber' helicals, pre-wound wire 'exposed spring' helicals, and 'splach' (tm) planar types.
Interestingly, some simpler TV type Yagi aerials have a bandwidth that includes 433MHz, but only use these on the RX end, or you'll exceed the ERP spec. limit
Have fun
Myk