Unfortunately not :(

There's a bit of guess work involved - two ways round it are :

i. Using Photoshop create and save the button - try importing the button and if it's too small/large, leave the import screen open, re-size in PS and click save. Then just click import again.

ii. Find a button on you existing layout about the right size (or that you can double to get the right size) and open the properties. Once you know the dimensions (in pixels) you can create a canvas in PS or Paint and scribble away to your hearts content.