I don't know that much about rent bikes down south. There are scooters and 250 cruiser style bikes for rent everywhere. Large bikes, not common.
In Chiang Mai you can rent Honda 400's and 750's from a couple of places. One being
Tony's Big Bikes
17 Ratevhamanka rd, Phrasing Muang, Chiang Mai Ph 053 207124
e mail firstname.lastname@example.org
Tony is an English guy, so the language is not a problem.
If you get up that way, you have to ride the Mae Hong Son Loop that goes from Chiang Mai, Pai, Mae Hong Son, Mae Sarieng and back to Chang Mai. The road is good and fairly quiet and has 1864 bends, and the scenery is fabulous.I've been over this road by busand thats a story in it's self. Mae Hong Son is a great little town. My favourit place in Thailand. Chang Mai is my favourite city.
There is also WWW.GT-Rider.com
that might be worth a look.
I was there in June year before last and the weather was OK, though that time of year there are often strong westerly's on the wetern coast (Phucket etc,) Not great for the beach.
Hope someof this is a help
Have fun Thailand is great
There's more than one way to skin a cat.