How to decide sprite size for isometric tile textures?

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I want to draw isometric tiles with arbitrary tile sizes. For example above picture there are 1×1 and 3×2 sized tiles.

The textures have arbitrary height because of decoration. a is the tile’s height b is texture’s height c is tile’s width. I can’t figure out what the sprite size should be when I lay these out on screen.

Maybe I should have exact tile sizes cover the whole texture without decorations (a == b). But still what the sprite size should be?