MacBook Pro early 2011 RAM

Memory can be different sizes.

However, Macs are very picky about the memory they use. If the memory you are using was not made for the Mac, then most likely you will have odd intermittent issues. If the memory was working in another Mac and the memory speed is the same, then the memory would be compatible, otherwise who knows.

If you want to purchase memory, then Crucial or OWC are recommended. Just make sure to use the tools on their web sites to identify the exact part numbers which are compatible with your specific Mac. With Crucial, they specifically mention "Mac Compatible" or "For Mac" on their product page (as do authorized resellers). Using any other part numbers than those recommended by the Crucial tool will likely result in problems even if the memory speed is the same even though it is still Crucial RAM. You cannot buy memory based solely on speed, or CL ratings anymore as there are a lot more specifications which Apple does not publish which must also be met for compatibility.

You can install up to 16GB. But either you mentioned would work. I'm not sure I would bother. A 2011 MacBook Pro is considered obsolete, can't run anything higher than High Sierra, which is no longer supported. Up to you of course.

That's not the RAM for that Mac.

You need PC-10600 (1333 MHz) DDR3 SO-DIMM modules