Hiya,
The measurement i was given, althoguh mine was already cut when i lifted the matting away, were from left hand wheel arch, vertical internal surface 42cm, and from the lip of the gavinised strip across the rear door, the first lip, not the external lip is 50cm, this gives you the centre of the pump... just measurged a fuel pump and it is near enough 13cm round, so i would guess a 16cm square hole form the centre measurements above would suffice.
Please note that you will cut through the support rib the runs the length of the tub.
Mine was already cut and i have had it open, whoe ever cut it, not only cut a great long hole in the floor, but it is also off centre, so part of the pump is still covered, but i managed to get the pump in and out as the hole was.
PLEASE NOTE: that i have never cut a hole in the floor my self. there appears to be a reasonable amount of clearance from anything you cuold damage with a saw, but take care.
Cheers