Silly question I know but whats a good length for the SAS cables to be?

The shortest you can find is .5 meters, which is plenty long enough.

I have found some for 0.3m

 

Is this too short?

I expect 0.3 m would be fine, nearly perfect in fact.  Check out the slack in the 0.5 m cables:

 

4220 pic

I have found a cable where one end of it has a right angle connector.

 

Will this be of any use/better use?

I would avoid it as the right angle cable my block you from using a second controller card.