I had the same thoughts before reading everyone else's posts...The 3 x 6 is more 'standard' and traditional, while the 4x8 would be a little more unique and bring a modern or industrial twist or zing to your kitchen...which I can like with traditional elements. Depends on what you want it to do. And Plllog does have a point about smaller tiles fitting in around things better, but I wouldn't necessarily let that decide it for me. Will one fit in the spaces using whole tiles, while the other will require cutting all the way along? (Remember to figure in grout lines) Even if I liked one better, if the other fit in whole, THAT could sway me to change my mind.

mtnrdux, I think you could really tell the difference between the Subway Ceramics version and newer subway tile if you had them installed side by side. The American Olean 3x5 I have from Lowes in my bath are perfectly nice, but they aren't quite authentic in terms of flatness - they have a bevel to them that reflects the edges and makes them look more modern. Schnauzer, both Emser and Lanka Tile make a nice white subway that's slightly less $$ than the Subway Ceramics version. You could also try Iron Gate tile.

I searched exhaustively for this for several months...sadly it doesn't exist for under $18/SF (Subway Ceramics, but they don't do colors). After that it heads north of $25/SF. If you can swing that, there are some nice options---there's an old thread listing several, including Heath (handmade but we went to look at them and they thought we could probably pull off a skinnier grout line than their recommended 3/8") and I think Sonoma Tile and a company in Portland whose name escapes me. If you're open to glass there are some more options, too. We finally gave up and used 3" x 6" instead.

Sorry I'm so late getting back to you. I think that oddly sized fixtures can be off-putting. Many home buyers are looking for an "ideal" and they can't see past that. If the tub is well made and the bath is located where it's most needed a shorter size tub might not be a problem. If the bath will be used by children then its size is less important, but children grow into teenagers. acm voiced a concern I had about the location of this bath to nearby bedrooms. My mistake in calling it water-resistant drywall. It looks like drywall but it's better referred to as tile backer board. It's most often 1/2 inch thick but I have seen a Hardie brand backer that was only 1/4" thick and came in 3 x 5 ft. sections. HERE is a link to more info on these. There are several types with differing properties. On toilet location, I don't think "obstruction refers to the nearest wall or fixture. I think it refers to something directly in front of the toilet. Looking at your floor plan an angled toilet would not have anything in front of it. There might be room between the toilet and the vanity for a waste basket.