Our shower hose is long enough to do this, no modifications made by us as to the length of the hose. I like having the shower head in the middle of the ceiling, plus it's about as high up as it can be (DH is 6'3").
I measured the distance from the top of the tub to the CEILING...it's 62" approx. The side near the center is about an inch higher than the side near the outside. Then there is a 2" drop to the seat at the back of the tub. Our curtains extend about 6" or more below the rim. The shower area is small enough that we wanted some extra length hanging inside the tub to make up for the elbow or whatever that might occasionally push the shower curtain outside of the rim and allow water to spray outside the tub. Keep in mind that the shower curtain ring attached to the ceiling is about 1/2", and also there are hooks that extend down another 1/2" or so, plus we had to connect ours with these round paper clip things (the grommets were too thick to fit into those small hooks) so that added another 1/2" or so.
The outside of the tub measures 23 1/2 by 31 1/2 approx. The inside measurement varies due to the shape of the tub; it is approx. 20 x 29 1/2. Two shower curtains will do the job and will overlap a bit.
I finally bought two heavy-duty white fabric curtains at WallyWorld. They came with a narrow, but somewhat weighty rubber strip that was sewn into the hem. I cut off the curtains, resewed the hem to its original width and then re-inserted the rubber strip by attaching a safety pin and then pushing the safety pin (attached to the rubber strip) through the hem. I didn't sew the curtains together, but just overlap them by one hook. After two camping trips totalling about 2 1/2 weeks using the shower daily, it was time to take them down and wash them in the washing machine.
I'd just buy the curtains, and then put them up, and then see how long you want them to be. Keep in mind that some of the grommets are too fat to fit into the hooks and you want to be able to remove the curtain easily if you want to wash it.
These are like the paper clips I used:
http://store1.yimg.com/I/quincy_1864_23033556
but I didn't wash the curtain with them attached. I just put them on after the last trip.
Have fun with your new TM!
Dee