I think that's a gorgeous focal piece, and leaves lots of room for adding. Perhaps pick one of the colors (or a pale hue of one of them) and do tile - maybe glass tile. That would provide a cohesive look and still allow that to stand out. I am concerned about the part under the hood...not quite sure how to resolve that piece, but I'm sure you'll get great suggestions from other Houzzers.

Can you get another custom piece for empty space? I agree with glass, choose a solid color that enhances, I'd also so use a larger glass like a 4x8 possibly larger. Beautiful kitchen!

Is that a glazed tile piece? Or some other material?

You are right to be concerned about scale. The cooktop looks tiny and the open space above it (future vent hood or ??) look even tinier. The tininess of these two features may or may not have been avoidable, but such a compressed, stinted area is not the right pace to put an eye-catching feature like your splatter-paint backsplash. That exuberant style needs an ample area over which to spread its generous appeal.

To fix.....no easy answers, sorry. I do NO recommend simply having the piece extended (klugey) or even remade (expensive I'm guessing). Here are chocies with pros and cons:

1) Place piece vertically, cut as needed. Cheapest and easiest but not a great look.

2) Remove 3 cabinets above backsplash, move backsplash halfway up, fill in with neutral tiles all around. Klugey and detracts from backsplash plus loses lots of storage.

3) If splatter look is important, find another way to achieve it. Maybe splatter-wallpaper or even painted drywall, behind glass or glass tiles.

Not having it all installed yet is giving you that empty feeling! Choose a glossly glass tile in a light color so it won't overshadow your gorgeous backsplash, perhaps the background color of the backsplash. I don't want to sound silly or overdramitic, but choose a tile that when you look into it, it looks deep, like water. That's what's so attractive about a glass backsplash, and your's especially since your focal piece is so exciting! :)))

Make sure you have the proper undercounter lighting that will highlight your special piece so it is the focal point. Change that out if you uave to. Use the tile you choose to complete the rest of the backsplash to fill in the area behind the vent hood all the way up to under the cabinet above.

sally has a great idea! add a small ledge shelf...you could actually put spices up there for cooking and add another piece of your backsplash art in the small space above.

OR you could do thin strips of stainless steel above it...either magnetic or not...have your knives up there in that space or other cooking utensils. If they are hanging down from the upper area, it would fill in the space nicely.

Such a pity... your splashback is beautiful, but it looks out of place at the moment. There are a couple of things you could do:
- cover the rest of the wall in a back-painted glass that matches the background colour (milky white?) of your splashback (similar to pic below)
- you could have it on the left hand side wall (the one with microwave+oven) if you can give up the plugs there, re-sized, obviously; choose something else for the rest of the kitchen.
- re-size and place it vertically in front of the hob.
- use it somewhere else (bathroom?) and consider new splashback for kitchen.

Good luck!

i love the shelf idea.

then do glass tile on the rest.
i would pick one of the bright colors from it! :D

I think that back painted glass in a color that you pull from the custom piece. This will not be as busy as glass tile with all those grout lines.

kitchen wall rack ideas for your space above........

Paint the bare bit the same as your cabinets, then put a steel utensil rack in it.

might sound daft: go to one of those places where you can paint ceramics [like this: http://www.ceramicscafe.com/] and paint a large plate to fit in the gap under the cooker hood. take your inspiration from your splashback. you could do one large and some smaller ones for other areas.

I think Sally has the best ideas she's right you just need to wait until your kitchen is finished , once everything's finished and in place it will all look different , don't rush at this stage ,I personally think it will look amazing left as it is , it looks great now :-)