Start at the door end of the matwell and measure from left to right then move to around the centre of the matwell and do the same measure then move to the top of the matwell and take the third measure. These are the left to right measurements so do not get them confused.
Next start at the door end again and measure from front to back then move to around the centre of the matwell and do the same measure then move to the opposite side of the matwell and take your third measure. These are the front to back measurements so do not get them confused.