How do I find out what size motor my 87 ford van has in it?

The engine identification tag identifies the cubic inch displacement of the engine, the model year, the year and month in which the engine was built, where it was built and the change level number. The change level is usually the number one (1), unless there are parts on the engine that will not be completely interchangeable and will require minor modification.
The engine identification tag is located under the ignition coil attaching bolt on all engines except the 6.9L and 7.3L diesels. The diesel engine I.D. number is stamped on the front of the block in front of the left cylinder head.

Also, with the VIN (Vehicle Identification Number) you dan find information about your engine: Ford VIN tools

I hope help you with this. Good luck (remember rated this help).

Look under the hood there should be a label that says what size it is,or take the vin number to the dealer and they can tell you by vin#