My DD is about to go on a 10 day trip with the school abroad. I'm going to buy a suitacase with a handle and wheels on as she will have to carry this around with her when getting off the coach and then onto the ferry (they have to cope with 100 steps apparently).
Obviously I want to get something big enough, but not to big as I'm trying to keep the weight down for her. I need to pack enough for 10 days. I've ruled out a 32" case as that's far too big, am looking at ones around 25" or 27".
Is this a trip with lots of specific kit like walking boots, jumpers etc, or just every-day clothes?
If just normal stuff then I reckon you have a good chance of it all fitting into a cabin-bag sized case (whatever that is). DD1 recently went to France for 5 days, and would have had enough room in her case for 5 more shirts, knickers and pairs of socks.
If loads of extra kit, or your DD will insist on 17 pairs of shoes, hair dryers etc etc then you may need to go one size up from cabin bag.
Why not get everything out you/she think she will need, and pack it into one of your existing cases to see how much space it takes up, and go from there? (Do this with volume not just 1 dimension)
Just to check - will she really need to carry her whole suitcase onto the ferry, or just enough stuff for 1 overnight? (in which case a small rucksack will suffice for that. She'll probably need a small rucksack as a day-bag anyway during the trip)
Luckily it's just every day clothes. Unfortunately, we're moving house in the next week so suitcases are already packed with things - also, there's a few items that she'll need that are already packed. Completion came up very quickly on us, so didn't have chance to have a practice packing session.
We've just come back from a 10 day family holiday - 8 (nearly 9) year old packs for herself and easily got everything into her 55x40x20cm (I think 21 inches long) wheely suitcase - but she only had crocks in their as far as footwear is concerned (and trainers on her feet to travel) and as it was a family holiday she also didn't have to pack her own bath or swimming towel. All her (summer) clothes for 10 days fitted easily, and in fact she packed more than she needed - she is in age 9.
She has that bag as we bought it to comply with the Lufthansa cabin baggage regulations and can very easily carry it up stairs etc. etc. fully packed.
If your DD needs to pack bulky things like a towel and spare pair of bulky shoes then one size bigger would be a good compromise - the 70ish cm length - about 26 inches.
Does she have to get everything into the one bag, and take it all out of the coach onto the ferry... as Teen says, a day-pack and a cabin baggage size wheely bag might be a better plan, as they might be more likely to leave actual cases on the coach for the ferry crossing, plus she will need the day pack for water and a raincoat or whatever on days out once there.