My guess is that since the one dimension is too large, it violates the rules. I'd remove and resize it to be below the dimensions so it maintains the aspect ratio.
nothing is stopping you from making it narrower though, the max width and height is 100/500 respectively, that doesnt mean it has to be 500x100 in order to use it
Yes, the maximum for the site is 100x500 as said already. I did however remake your sig to be within the rules. If you wish to use it, here is one for the dimensions 394 and 100, as for some reason it wouldnt format 500x100 correctly..