Is the flowmaster muffler model #42541 to small for a toyota tacoma prerunner?

The 42541 has a 2.5" inlet and exit pipe, which is plenty. Which engine do you have? The 2.7L comes with 2.25" diameter pipe and the 4.0L comes stock with 2.5" pipe, so no matter which engine you have, it'd be fine from a backpressure standpoint. That muffler would work best if the rest of the exhaust system is 2.5" so that you wouldn't need to use pipe enlargers and reducers to connect it. No problem on back-pressure--the inlet and exit sizes will be either exactly the same or close to the stock size. Just check the muffler length and exit/inlet offset to see how it matches the rest of the system to see if you'd need to modify it.

What do you know about backpressure?

You test backpressure by PSI in the exhaust pipe, and/or use manifold vacuum readings under full throttle to see if vacuum increases under load, indicating a too-small exhaust system.

Backpressure is bad, it is always bad. SOME is a necessary evil since CFM is a constant given a specific diameter of pipe, so you may have some that will limit upper end performance, or you will have little at higher RPM's, which hurts scavenging at low RPM's.

42541 Flowmaster

Leave it alone.