Were the store unable to help? It's really difficult to do online as you need to know things like the rim and hub dimensions, and whether it's a rear or front wheel as they will have different spoke patterns.
I've tried looking online (https://www.decathlon.co.uk/tilt-700-folding-bike-grey-id_8300175.html) and managed to find that it's a 20" rim but there are no dimensions for the hubs.
If you are able to measure the wheel yourself, then this website might be helpful:
Sorry that I can't help more than that.
Right, after some basic research - I removed one of the spoke and measured it. It appears that the size is 178mm.
Truing the wheel was a bit of a pain but now the wheel is back in action!