gearbox fluid

Hi guys, I'm new one here but I've been lingering for a while. I've got an A170CDi which is fully auto, and I've just bottomed it out on my drive at home. Realising what I'd done I stopped immediately and got out to check for any damage, and it's sprouted a leak from just behind the front...

After my W210 (320) auto 'box having a few gear selection bumbles, I wanted to check the oil level/condition as a start, but instead of the old long gearbox dipstick of the 124's, there is none! Please may someone tell me how long a piece of wire needs to be to act as an accurate dipstick? (With...