Flash and RAM are different aspects, better not to conflate them (but yes, you want 'decent amounts' of both).
The UBI partition on this device seems to be 252 MB, but I have no idea how large the UBI itself defines the UBI volumes (modern devices are typically designed around dual-firmware setups, which halves your space from the get-go, with more space 'lost' to other vendor partitions/ volumes).
EDIT: this suggests ~65 MB