Oops- and there's ~14" from the existing splash to the bottom of the cabinet.

No way am I gonna touch this one - too lazy. That would discourage me to know. Why measure, if you're using shards? Just start breaking and gluing. Of course, I see your point, if you want the colors in your stash to be evenly scattered throughout.

I have so many questions...
Add to the above question: What is the best way to 'break up' this space with the hardiebacker boards?

Whatever size you're comfortable with. For example - on the first soffit that's 38" X 5" - why not start out w/that size, leaving screw holes in the backerboard. When you're finished grouting, turn it over, drill those holes back out that have filled w/grout and put it up. Depends on the area in which you'll be working, but if it were my project, I'd do all the uppers the same way - cut the size to fit the area, drill screw holes and mosaic away. On the backsplash, I'd probably divide that into two or three pieces. Ask away. We aim to help.

So straight (square/rectangle) pieces are better to work with than say, "L" shapes? How do you hide the "break-lines" if you do it this way- another slap of grout along the breaklines afterwards?

Never done this but there's someone on here that's doing a wall, using backerboard - sorry I can't remember right off, and she would be a better one to answer that one. The way I hid the break lines in "Shea's Tower" was after I put them together, I just put in some silicon and stuck tesserae in to hide the seam lines. It didn't completely hide it but from a distance, it's not quite as noticeable. These are six chimney liners and they were uneven, so there were gaping gaps between each section. That's what I'm doing on the second/third photo.
"Shea's Tower

Maybe I'll just set them side-by-each when I get close to the edges so that there's no break at all in the design. Then (hopefully) the designed sections will just flow through each other with no real 'line' at all. Yup, that's the ticket...I'll do that.

Don't ask about the design...it's not even in my head yet.

I'm doing the wall and what I'm TRYING to do is overlap the design a bit so there is not an obvious seam. How successful that will be is yet to be seen!!

Maria

I am very new at this but what if you cut the edge of the backerboard uneven...kind of like a puzzle...with slightly curvy edges so they fit together like a puzzle???

Ooooo! I like that idea, lnl!

MM, I am NOT a math wizard, but let me give you my thoughts on your numbers. If I round off your numbers for an estimate, under one cabinet, you have 3 ft (38") X 1ft (14") and that would be 3 square feet. That's for your largest area. So I think you might want to recalculate your numbers. Did you divide your square inches by 144 inches? (that's what my Engineer DH says you would need to do to convert your square inches to square feet).

My other thought on your backboard, is to consider placement of your holes so that you have support such as in a stud. This will be a heavy piece to hang.

Let me know if that makes sense.

I knew I was doing something wrong...nope, divided by 12- DOH!
Again, not smarter than a fifth grader...