Does rim size make a difference to chain length? I plan to upgrade my bike to 1x10 from a 1x7. I also want to learn how to build a wheel. 

In order to archive this I need to change the hub on the rear wheel, but I am waiting for the hub I need to arrive.

I plan to use my spare thin 650b wheelset then install my chain so I can ride in the meantime. Does the difference in rim size affect the chain line so much I won't be able to use my 700c wheel once I have built it?  

I plan to take it to a place in stockport so I can be supervised.
The rim diameter of the wheels has nothing to do with the required length of the chain on a derailleur equipped bike.

That does make a difference is the size of the chainrings and cassette sprockets. If you are using the same cassette on both the 660b and 700c wheels then the chain length will not need to be changed.

You dont say when in this process you are switching from 1x7 to 1x10. You will of course need a new 10 speed specific chain when you upgrade to 1x10.