I just purchased a 1970 beetle, how do I tell what the engine size is?

1970 came with a 1600cc single port but, it didn't go dual-port till 1971. That said without listing you block number(under generator stand) it's any ones guess what's currently in your car. Compare it to this site:

http://type2.com/bartnik/enginecodes.htm

Also keep in mind that it could have been rebuilt and the only way to be 100% sure what you have is to take it apart and take measurements.

It came with a dual port 1600cc engine and made about 56 horsepower. Under the carburator is a gray color T pipe at each of the bottom ends are rubber boots that connect to aluminum manifold extentions that feed air and fuel to the cyls that is a dual port. If there is gray tubing to each cyl head it is a single port 1500cc or smaller 1300cc engine 54hp and 52 hp. Single port engines stand up to heat better down mexico way the bug was made up until1976 with single port motors.

Theoretically, the engine capacity should be stamped on the cylinder head. Either that or the model (eg. the 1303 has 1300cc etc.). Or in the logbook, which I assume you don't have otherwise you wouldn't be asking. Hope that helps!

goto the hood and read, or check the volkswagen web site

Open the hood and read.