Set size of share folder by samba

I have share a USB (fat32) by Samba to do this I have create a mount point about USB.

Now I have a new resource on my network (openwrt) and inside it a folder with USB content.

However I'd like have 2 folders: one to access to all USB content, and one to access to a sub folder of USB and limit max size of this folder.
Is it possible? How?

I think the fast way is create 2 partition on USB and then create 2 mount points and 2 share folder on samba. But is there a way without create a partition?

As far as I know, FAT does not support quotas; I guess you will have to use partitions.