First off I am new to pocket watches, both stem and lever set. My question concerns being able to purchase a watch case, listed as an example, either a size 16 or 18. Will the 16 size watch mechanism I have fit into the 16 size case I am looking at to purchase?
Is there any rule of thumb or case markings to help with knowing what case will fit which watch mechanisms. From what I have read, watch companies did not make cases for the watch's they produced, but used other company cases for their watch.
Sometimes a watch case is listed in a specific size. Does this mean that the watch case will hold watch's of that size?

Size 16 case will fit size 16 movement but
make sure your case is for lever set watch or stem set watch accordingly to your movement

Peter is very correct. Some cases have a "sleeve" in them that allows for pendant setting watches and some cases don't have a sleeve so they would only work for a lever setting movement. The lever set notch cut out in most cases for 16 sz watches is at the 5-6 minute mark, however a few cases have notches cut out at 7-8 minutes for some movements. Some have a lever slot cut out at 55 minutes for Waltham movements.

There are a few early 16 sz movements that required special 16 sz cases because they had over size under size pillar plates. There were even a few 17 sz movements out there that require a 17 sz case !!

If you will come to pitfalls and ask your question that may arise about certain cases or movements, I feel sure the forum members can help you with anything that may come up.

regards,
bb