MM, I would strongly suggest you do one or two small projects prior to doing your backsplash if this is your first project. You will learn through those projects, what are your preferences in mosaicing.

Personally, I think a variety of sizes and shapes add to the interest of the mosaic piece. You might want to peruse some websites to study other's artistic styles too.

Do some research into ancient wall and church mosaics - you'll see that they covered dozens of square feet with TINY tesserae. Don't get lulled into the bigger is better mantra - sometimes bigger is duller. And don't think big tesserae=big grout lines. There's nothing that screams amateur louder than large grout lines and rivers.

I second the notion of doing a few smaller projects before jumping into the PERMANENT backsplash idea.

d3- thank you; while I likely won't do any small project beforehand, I fully intend on laying out this project before I glue down ANYTHING. I plan on HAVING a plan...on paper first, then drawn onto the hardiebacker, then just laid out (even if haphazardly) onto it so I know what the ultimate look will be.
I like to think I have a "good eye" for design, and can usually tell immediately if something looks whacked.

I very much agree with D3bbi, also....in regards to doing at LEAST one smaller project first. It's not just the layout - it's the whole process including grouting. You learn so much from each project you do.

Wench is right for a number of reasons... not the least of which big pieces of plates don't go near as far as the same plates cut smaller. It's a matter of personal preference whether you mix up different sizes, what makes a piece stand out is the grout lines being a consistant size and the tesserae haveing a relationship - flow... uh??? I didn't see that in the backsplash you posted before, it all just seemed like hundreds of individual pieces that were not arranged or related to the next pieces... take a look at the difference in that example compared to Klinger's in the favorites thread

Ditto on the "relationship". I made a GOG with 6 panes. Used varying sizes of a geo shape in all of them. That was ok. However, when I filled in the background, I ended up with much smaller pieces on one pane and to me, it ruins the whole thing by ruining the flow.