What size glue gun to use

Glass marble beads do not need a lot of glue, so a mini would probably be alright for the one project. If you plan to do any more crafting or gluing with a glue gun, you might want to consider a larger one. Otherwise go for the mini! Let us see a photo of the mirror when you finish?

I think a mini gun would be fine

Sometimes using a glue gun for marble beads doesn't work well. The glue may not be made for glass beads and the glass beads will pop off--leaving the hard glue behind. A mini glue gun would work fine--but get the right glue for your project.

I just did something similar to my bathroom mirror. I used the glass stones you get from the dollar store. I wanted to be sure they would stick even with the bathroom getting all steamy. I used the bigger glue sticks with the bigger glue gun so you can work faster. The smaller glue sticks would take more time.

I like the mini gun, I feel I have better control over the amount of glue I use

I have both sizes and I like my mini gun better. It doesn't take as long to heat the glue because the glue sticks are much smaller. Try to get one with two temperature controls. Higher temp would be faster gluing small things like the glass beads. The down side of the mini is you have to load glue sticks more often.

Go to a craft store like Hobby Lobby or Joann Fabric. They have the best selection of glue guns and glue for your project. Walmart also has a good selection. You will be able to read on the glue packages what will work best for glass beads.

I think it's dependent on you. I have both sizes and do find that it goes faster with the larger one for bigger projects but like the mini ones for precision work.

I would think a larger one would work best in this case so you aren't spending time refilling the glue gun. The mini-ones work best for very small projects.

Don't use a mini gun. For the amount of "hold" that you need, you need a larger glue surface that you'd get from a regular size glue gun. I wouldn't use this, though, if you are using glass marbles, which are heavy. I would use E6000.

Yep! I used a glue gun on a craft I put outside last summer and it fell completely apart in the sun. I used the E6000 and it held up all summer in the sun and heat!