Well I finally found this one manga called Ebisu-san and Hotei-san and I really liked it. I've been searching for a long(ish) manga about yuri/shoujo-ai - romance, but only came up with one shots.
So could you please tell me some other manga that are somewhat long with this genre!
Thank you in advance.

Yeah unfortunately most yuri manga (or the ones that are scanlated, anyway) are just oneshots.

Perhaps you can try Octave and Aoi Hana. Both are currently 5 volumes long and still ongoing.

Also check out Girl Friends. It's finished at 5 volumes.

Is one volume long enough?

Some two- or three-volume manga:
Pieta
Strawberry Shake Sweet
Heart-Pounding Excitement at Mononoke Girls' Academy
First Love Sisters
The Sword of Paros
Strawberry Panic!
Honey & Honey (though this is more autobiographical than "yuri")
Hanjuku Joshi
Gokujou Drops
Manga no Tsukurikata (ongoing)

Sasameki Koto is another fairly long ongoing manga.