I might have destroyed my crank arm :(

The stock pedals on my new Cube Attain were aweful and started falling apart within a couple of weeks.

I bought these to replace them:
http://www.chainreactioncycles.com/wellgo-v8-copy-flat-pedals/rp-prod563

When I fitted them they seemed to be screwing in tight but I just carried on :| now one of the pedals has fallen off and when I tried to tighten it the hole was too big and the pedal spindle was moving in it freely. I'm not sure if these pedals were too big or if I just knackered the thread.

So my question is do pedals come in different sizes? Should these pedals have been ok on my Shimano Sora crankset?

How do I fix this? Do i need to replace the crankset ?

sounds like you crossed the threads and fucked the cranks, that is all

knackered the thread

I reckon so.

A Helicoil insert is one way out, but given the cost and effort you're likely as well off getting new cranks.

Hey-ho, but it's better to learn from (relatively) cheap mistakes.

You fucked it. But good job only on sora cranks and welligo ped's

Next time, if you feel it's getting to hard to turn then stop. Use grease and keep all threads clean

Next time, start threading the pedals in with your just your fingers. You should be able to tighten them in almost all the way before you even need to start using any tools, and also avoid any chance of cross-threading them

The old 'turn it the wrong way while pushing in till you feel a little click, then start screwing in' always works for me.