As a person with an education in art, I can attest to lugging things around (across campuses as of late) and I know that having a bag similar to what you are describing would have been a dream! Not only do art bags tend to collect things in the bottom of them like pencils, or what have you, which mark up the sides of the inside of the bag. (Things will fall out of their cases from time to time.) But also having compartments to store canvases, paper, or other vertically oriented things and be able to carry them easily is always nice.
The size of the bag may seem big, but for an artist, it really isn't that large. My case is 48" wide x 36" tall. (I am also 6'3" tall)
While most of my projects don't come close to fitting in that space, I had a few that did which is why I bought that size. Thus, if she needs a 16"x18" bag, go ahead and do it. You may even want to round it up and make it a 20"x 24" so she can grow into it (that is if she is really into art and you think she will stick with it for a long time.)
So, to answer your questions directly:
1: Yes, I think a messenger type bag would work perfectly for a budding artist and of the size you mentioned (maybe even a little larger if you wanted).
2: Instead of using laminated fabrics, I'd use something like canvas or another ultra durable material. It washes easy, lasts for forever and will withstand the wear and tear of the artist's life. (I know my case is made of canvas and I've had it for 10 years. Still working great!) Plus, it is super eco-friendly. The final bonus, it doesn't really snag or tear. The laminated fabric looks good for a while but with sun exposure it becomes brittle over time and seems to break down faster making it crack and thus easy to snag. Then you may just want to chuck it. I'd go with canvas; she'll likely have that bag for the rest of her life.
3: The conundrum of 2" vs. 3" - when in doubt - more room is better than not enough. Plus if you made it with Canvas, it will be soft. So it will expand to its full capacity as she puts things in there.
Good luck with your project, hope this helps.
PS: If you want to make your bag extra durable, you can put leather or vinyl on the bottom of the bag so that when she sets it down at school or other places it doesn't tear up the bottom.