Raleigh 300 Crank Bolt specs for replacement I have a Raleigh 300 and the crank bolt fell off and was lost. 

I have no idea how to find out what size crank bolt I should get! 

Can anyone advise?
You lost one crank bolt.  So there's the correct bolt on the other side of your bike.  They should be identical.

Take it out and go to the bike shop with that.  Its likely you'll need a 14mm socket to remove the bolt.

Talk nicely to them and you might just get one out of the parts bin, specially if you're a regular customer or buying other stuff too.  These bolts generally don't wear out.