Roman coin to identify, please.

Also, does it look to be genuine or a copy of some sort ? It's approx. 27mm

For me it looks like an Antoninianus (Constantius I) 305-306 AD. A better picture would help.

Looks like a centonianus of Maximiam, forgot how to spell his name.

My research now says that this is a Galerius (Maximianus) A.E. Follis, from 295-296 AD.  Cyzicus Mint.
Width is right at circa 27-28mm.  Weight about right at 10.6g.  Flan shape is right. Metal is right.
The lettering on the head is correct GAL VAL MAXIMIANVS NOB CAES,
The back also fits with the image of Genius standing naked with the cornucopia and patera. The lettering is also correct GENIO AVGG ET (break for the head) CAESARVM NN
and KA at the very bottom under his feet.

Well, you are the one that see the legend better. If on obverse is GAL VAL MAXIMIANVS NOB CAES and on reverse is  GENIO AVGG ET - CAESARVM NN then you're right. By the way the normal weight should be 9.2 g but the coin is a little bit oxidized, so this may be a reason why the weight is more than normal.

http://www.wildwinds.com/coins/ric/galeria_valeria/t.html

Thanks Wildwinds.

I've found confirmation on your site.
Matching image: -
http://wildwinds.com/coins/ric/galerius/_cyzicus_RIC_011b.jpg

And Text description.

Cyzicus RIC VI 11b

Galerius Maximian, Cyzicus, 297-299 AD, AE Follis. GAL VAL MAXIMIANVS NOB CAES, laureate head right (large head type) / GENIO AVGG ET-CAESARVM NN, Genius standing left, modius on head, naked except for chlamys over left shoulder (falls low), holding patera from which liquid flows and cornucopiae. KA in ex.