Just to let you know, the lumps that are supposed to surround the flats are supposed to have 2 'F's in the name, not 1. And you should try using "Save as Doom Flat" for flats that are 128x128 or bigger, so they won't appear strange in-game.
The height and width of a flat must be a power of 2 and be equal.
The width of a texture must be a power of 2, but the height can be any value up to 255. If you want to make wall textures with a size of 256x256, 512x512 or 1024x1024, etc, use multiple patchs for them.