The smallest my pattern has ever been was 8700x13500px...and even that's a little low res for my perfectionistic tastes.
That said, the muscle map itself can be upscaled usually more easily than the rest of the pattern since it will have so many other elements overlaid on it to cover any slight issues with the smoothness of the transition between shades of grey.
I'd still much rather have a higher res file to start from, if I was you.
-Nick