The Kickstarter you linked doesn't use pixel art at all. Sprites can generally be any size or shape you want them to be, as can tiles. Sure, in the old days, tiles needed to be a specific square size, like 32x32, but these days, you can use whatever. I've seen games where the tiles are rectangular, but can be overlapped.
Anyway, the most important info is what Ai told you about mockups. You never ever choose these things ahead of time, that's silly. What you do is you make a mockup, then you make another and another. After 5 mockups or so, you see what works, what doesn't. Make a mockup with small tiles, make one with large tiles. Try it all out.
I will say one thing, 64x64 tiles sounds crazy to me. That's a LOT of real estate. But hey, prove me wrong, make a mockup and share it, we can discuss what you seem to be doing wrong/right.