Lemonmuncher, I always work in power of two sizes (8x8,16x16,32x32,64x64,128x128,256x256 are the main ones I use for environments) when doing environment tiles. It's the general rule of thumb for games art (I'm almost certain Construct 2 complains about not liking non power-of-2 images), so I always stick with those sizes. It also helps keep all of your art tiles aligned nice and neat to a one-size-fits-all grid.
You don't need to keep it all the sides equally sized though, as long as you keep it a power of 2 number you'll never lose the grid, and Construct 2 won't whine at you about your images being non power of two.
For example, these assets from the jungle temple tileset:
<img src="http://i.imgur.com/9VG4qik.png" border="0" />
<img src="http://i.imgur.com/ECbuFgI.png" border="0" />
<img src="http://i.imgur.com/69nW9DR.png" border="0" />
You don't need to fill the whole image like I've done in these examples, but I always tend to keep the canvas size a power of 2 so that I don't get any weird errors, and everything plays nice with the grid.
I hope this helps!