In reply to Charley17:
I'll have my two bobs worth as well.
19 and 16cm screws are probably your 'typical' length. Short screws are fine for hard thin ice but if you want a screw for all situations then I would stick to the longer lengths (remember, you can always larks hitch the shaft if the ice is not thick enough). As mentioned above, 22cm are generally a bit long for if you start leading but I always carry one for abalakovs. Always good as well for 'must not fail' ice screw placements as well (below cruxes or run-out thin ice sections, etc). I would just get a 16cm and 19cm screw to start with, which will be fine for crevasse stuff.
As mentioned above, spend the extra and get some screws with the winder handles (eg BD Turbo screws) as you will able to place them quicker. Not so important for crevasse rescue but if you start leading on steep-ish ice then you will find the winder makes a big difference when placing with one hand.