The actual file size is pretty much unimportant. A lot more important are the dimensions of the actual texture. Very old hardware will require textures with dimensions that are powers of two or maybe even quadratic textures. As for today, I'd say using a maximum of 1024x1024 pixels is pretty much save, although 512x512 pixels can be even better. Use a size that's as small as possible while still keeping your number of textures (or more specific: the number of texture changes) minimal without using too big textures.
Don't think they still sell desktop graphic cards that aren't able to handle 1024x1024 pixels, but for mobile 512x512 pixels sounds a lot better (ignoring the fact there's no port of SFML yet).
SFML tries to handle those limits for you, but if you use - for example - a texture being 520 pixels wide, SFML might be forced to allocate and use a texture that is essentially 1024 pixels wide (as 520 pixels obviously won't fit into 512 pixels).
