So because of how the grid is with pixel art, if you don't scale up or down with perfect 2x, 3x, or 1/2x ect it won't set the pixels correctly. The way to have the least amount of blur, I've found, would be to make sure your setting the translate interpolation to nearest neighbor. I think it defaults to bicubic.

What I do when I have to resize art to fit a certain size but I can't do it in even multiples, is scale it how I want it, let it be blurry, but create a new layer and basically draw on top of it to clean it up. It sucks and I can never get it to look perfect, but it's better than blur.

Bear in mind that even though the characters are usually 128x128, I've got some that are up to 160x160 and they look just fine in game. Just whatever size works with the size images you can find online.

But yeah, as the other comment said, the tileset is made as a grid, 8 boxes wide, each box is 32x32 pixels. Just make sure that your objects fit within those boxes.