Since you scaled by 2, you can un-scale by 1/2
// first draw
ctx.strokeRect(5,5,25,15);
//scale
ctx.scale(2,2);
ctx.strokeRect(5,5,25,15);
// un-scale
ctx.scale(.5,.5); // unscale by 1/2
ctx.strokeRect(5,5,25,15);
Another example, if you had scaled by 3 you would unscale by 1/3
// scale
ctx.scale(3,3);
// unscale
ctx.scale(1/3,1/3);