Thanks for asking about it.
You are right, you can not resize, draw or use the lasso tool on any reference. We are actually going to do a resizing mechanism, so in one of the next alpha releases you will be able to do exactly that.
For now, I suggest you fix your reference image elsewhere (photoshop or other image software you might have). So make it fill the HD frame as you want it, then save it as png and import it again into Animation Paper.
I hope this helps.