I absolutely agree with cpccarolyn. Since you can't see into the future in terms of what will look dated, and what won't -- (or if it's dated, will it then be considered "classic" or "trendy"?) you should use the colors or sizes or textures that you like. Regarding resale, in our neighborhood, every house that gets bought gets completely redone anyhow, so who knows?

I'd honestly be more concerned with how the angle is handled than grout. With multi-material mosaics each one can cut differently as each material is different. They should be able to get an angle but it's never the easiest thing because the tiles have to meet & match perfectly. I would expect that would mean 1/2 tile on each half of the corner -- so it looks like a single tile. I'd surely ask in advance at Bridge. Making that come out exactly is no easy thing. They might want to start there and work towards the corners where a different cut might be less noticeable as it's adjacent to molding or dying into the wall. I learned this because I had a 4 color tile design on my splash and it was handmade tile and not even in size. It also had to turn a corner. It might possibly be the setter wants to put a vertical line of pencil pieces over the corner and tile up to it. That slightly blunts the edge. They could use pencil pieces from the range (if they exist). Or, they could do a line of single mosaics of one material cut off the mesh (not as nice but acceptable if done well). Only way to know how it will cut is if someone had worked with that exact mosaic before. But I wouldn't want to have the setter come to that realization with wet thinset on the wall IYKWIM. Or think a crappy looking or uneven joint there will be ok when it certainly won't because it wasn't discussed and they started at the back. If someone tells you bullnose should go there, tell them you think it's too "bathroomy" and what else can you do? Might be a good idea to loop in the contractor in advance. He likely isn't focused on any of these issues. I'd ask if he could look at the time and the angle and tell you how it will look and if any special tile pieces might be needed.

My personal preference is for wall tiles in the niche, and for the niche box itself to correspond directly with tile dimensions/grout lines (which is admittedly quite tricky to do at the construction stage, but aesthetically I just hate seeing niches randomly placed without reference to grout lines). Here is a recent project with 24x24 tiles (requiring one cut of the tile as a 24x24 niche would have been too tall). The niche is framed with Caesarstone pieces (this is my usual approach as I typically also use matching Caesarstone pieces on the top of the shower curb). Here is another example of mine, in this case with the niche framed in stock 'big box store' Carrara marble door thresholds. Again, the niche is the same size as the tile:

Larger is not out of proportion with the 1" tile. It SHOULD look larger. I would use a 4 or 5 x 10 myself...