At these large sizes, I feel like there's no point in using pixel art, you may as well go for "HD" art. Putting pixel polish into art this large is a big time sink with hardly any pay-off. Don't forget that the sprites also need to be animated, and each frame also needs pixel-polish if you do pixel art.
As for your actual tile size - you should choose what best serves your gameplay. Random people on the Internet with no idea what your goals and gameplay are can't really help you make that decision. For some games, large tiles and sprites work great! For some, smaller ones are a better fit.
For example, if you need to have a very large, fixed field of view for the game to accommodate your gameplay, it makes sense to use small tiles and pixel art. If only a small part of the world should be visible, or if you want the player to be able to zoom in/out, then high-res non-pixel art tiles are a better fit because they scale better. On the artwork side, if you plan to use skeletal rather than frame animation, HD art will give you better results than pixel art. Consider such factors in your decision.
Another thing to consider is that cosmetic tile size doesn't mean as much as you seem to think. A game with the same character and world scale as your 128x128 example could be made with 16x16 tiles as well, each cosmetic tile just would have less of the world in it. Instead of worrying about your tile size, try making some mockups sketched from scratch (without using any of your existing assets, which would bias you towards your existing scale) to determine the overall character, world, and gameplay scale that suits your game. After that, you'll be able to make an informed decision about appropriate sprite and tile sizes.
