They said that it wasn't possible to do that as it's a shared hosting :/
Yeah unfortunately many shared hosting companies have a lot of limitations.
In case you do look for a new host, be sure to ask them before hand if such values can be changed or what the default values are.
If you need a recommendation: I'm very satisfied with stablehost.com.
In case your forum is big, you may want to consider getting a VPS instead of shared hosting.
Yep, exactly or purchase a different plan that allows for the proper set up. Although if it were me I'd go to another host. There are lots of good ones that do not cost much that do not have a problem in helping with these types of matters.