Most threads used in bag making will be polyester or nylon thread as it's much stronger than cotton thread and doesn't shrink.
Lighter weight fabrics need a lighter thread such a multi purpose moons/coats or guttermans polyester thread. These threads are usually measured in denier and a standard multipurpose polyester thread is 120d it is suitable for most machines including domestic machines. The higher the denier number. the thinner the thread.
The best needle size to use with these threads for bag making is 90/14 or 100/16 depending on the thickness of your fabric your using.
When using heavier weight fabrics such as upholstery fabrics, faux leathers and leathers in your bag making you can use a heavier weight nylon or bonded nylon thread. These threads are usually measured in tex and tex 45 and 70 are pretty standard in bag making.
Some domestic and vintage sewing machine can accommodate a tex 45 thread but most domestic sewing machines will struggle with tex 70. An industrial machine will be needed to use tex 70 thread. The higher the tex number the thicker the thread. The best size needle to use with tex 45 thread is 100/16 or 110/18 and with tex 70 is 110/18 or 110/19
I have found it is good to match the thickness of the thread to the thickness of your fabric, for example if I use a tex 70 to sew the linings of my bag if I am using a lighter weight cotton. I find that my seams with gather and they dont sit right. If I switch to the correct thread I get a much better finish. And likewise if I use tex 70 on my industrial machine for thicker fabrics on thicker seams they are much stronger and my top stitching is much more noticible.
I hope this post has been helpful please feel free to comment with any questions.
Until next time.