If its a type 1 based engine then at most it will be a 1600, if it's a type 4 based engine it could be up to 2000, in stock form. If not stock then who knows! You can go by the engine number, if it has never been rebuilt, that would be good, but it is very likely that it may have been rebuilt at some point in it's history and the only true way would be to take a head off and measure the bore and stroke I'm afraid.
This place will give you an insight in to stock sizes for engine numbers. Click here