How can I set collision type of part with mesh in it?

water part used mesh scale to dodge size limit, but it just occured collision bug. It just non collide even I just set collide on.

how can I set collision type so I can make it collide and get touching part with script?

Pretty sure the max size of a part in roblox is 1024x1024. If you add a mesh to that, only the 1024x1024 part will have cancollide on. The extra part won’t collide and I’m pretty sure you cant change that.