Bobbin size-Problem

Well, my husband lost my manual to my sewing machine while moving back into our house. I am a newish sewer and so I apologize in advance if this is a dumb question. I am going back to that old "there is no dumb question..." saying to justify my post.
I have a brother pacesetter that is about 5 years old. I cannot  find anywhere on the machine the model number and I cant find it on the brother website. I am out of bobbins and when I take my bobbins to the local craft store I can find ones with the same diameter but that are deeper. They still fit in my machine but I seem to have tension issues. Any suggestions? Does diameter of the bobbin matter more than depth? What about plastic vs metal?
Thanks
Kirsten

In my experience, plastic vs metal isn't as important as horizontal vs vertical feed.  I think drop-in horizontal bobbins do not hold as much thread as vertical bobbins which is why the "deeper" bobbin you bought isn't working right on your machine.  Do you drop-in your bobbin from the top or do you have to pop your bobbin in a case and snap it in place in the front of your machine? 
If you drop it in, look for the shallow or size "L" bobbin NOT the class 15 which are usually sold at most chain craft stores.  You might try a dealer who would know more about your machine and they could recommend the correct type. 
You are correct though, the wrong bobbin can cause havoc with your tension and stitch quality. 

I drop my bobbin in from the top. Thanks for the help!