For an Aspen LE-1, Tamarack LE-1 and Aberdeen LE-1
Generally, the recommended size is a standard 48" wide by 48" deep hearth pad. If you want the smallest hearth allowed use the information below:

On a straight Wall:
The Aspen LE-1 is about 21" deep so if the stove is set at its minimum clearance from the wall (7") you will need a hearth at minimum 44" deep (16 in front, 21" depth of stove, and 7" behind the stove).
The Aspen LE-1 is about 26" wide so you will need a hearth at minimum 38" wide (6" to the left, 26" width of stove, and 6" to the right).
In a corner:
Use the same clearances from the illustration above (6" on each side and 16" in front) noting that with the stove set in a corner at minimum clearance (5") the hearth will extend from the corner about 57" into the room.