Just putting on larger tires won't get the job done.  There isn't room in the well for any substantial increase.  If you add an inch in diameter, you only lift the trailer half that amount, ie 1/2", which wouldn't be appreciable.  

The only way to gain added height is to have a lift kit installed.  This is essentially a 2" or 3" spacer of steel welded into place, between the axle mounts and the frame.  Another way to raise the 13' Scamp would be to remove the "up" axle and replace it with a "down" axle gaining perhaps 3" in lift that way as well.  Of course, a combination of the two would also provide lift.   All the while it is good to keep in mind that the while the body is being lifted, the axle is remaining virtually constant in relation to the ground, thus appreciable ground clearance for boondocking has not been improved.

In this market segment, having 15" tires and a lifted camper is much more typical of Casita 17 footers than of Scamp 13's.