I'm trying to get to grips with a piece of music which has LH middle C to an Eflat and I'm having real trouble getting my little finger in the right place.... Do the keys on clarinets vary in size or do I need to grow my little finger?
I've been practising and practising but it seems a real struggle to get a nice transition between the two notes.. Does anyone else find this a problem? I've only been playing since xmas

I'm guessing you mean the C above middle C to the Eb above that?
Yes, the keywork on clarinets does vary from model to model so your instrument may not be the best fit for you.
Have you got your RH thumb a little too far under the thumb rest?

Apart from that it is a tricky move to make tidily, I know as it's part of a technical exercise I'm learning for grade 5 and I still sometimes don't get the coordination quite right.
Try to move your little fingers as little as possible.