The first step to determining size is to measure the length of the cylinder head. If it's under 24" it's a small block, over 25" it's the larger block.
The small engine would have been either a 218 or a 230 depending on the year. If the flywheel mount is visible you can count the flywheel bolts, four bolts is 218, eight bolts is 230.
The bigger engine runs some similar sizes, but not the same, but also comes in up to 250 or even 265 inch sizes. Measuring the stroke will determine this (as outlined above by Beater) and references to engine sizes can be found on the internet in various places.
What I refer to as 'small block' and 'larger block' you will find referred to by bore size in those places.
