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.
Just take it to a bike shop. They'll probably be able to replace the bolt there and then, at a reasonable price and much easier than trying to figure out and source the exact right bolt yourself.