I'm just going to add my suggestions (many have been posted,but I'd like to second them).
At least a 2.5 G ,though if possible go for a 5G (approx 30cm square).If you might want to get second Betta in the future get a 10G and you can always divide the tank into 2 sections).The myth about Betta's living in puddles is rubbish,they can survive in a puddle but it doesnt mean they are happy (if I locked you in a wardrobe and fed & watered you,you would survive but you wouldnt be happy..nor would the police
Filters are benefical to the fish & water quality,you can get away without a filter in small tanks but this will require more frequent water changes (and larger % water changes),with my 5G (live planted) Betta tank I do a weekly 50% waterchange (1 bucket) using a gravel vac.
You must maintain a stable temperature of between 76-82 deg if you want to keep your Betta at its happiest.I dont know much about the weather/temp where you live (I live in the uk in a 160 year old house,which is bloody freezing in the winter so I have a heater which is in the tank all the time
These are a must for a happy Betta,they can hide amongst them and explore them to keep themselves amused.Live plants are benefical to the water quality (plus i think they look better) and some are easy to keep (I have my tank light on 8 hours a day) If you go for artifical plants its best to go for silk ones,some of the plastic ones can have sharp bits that can rip your Betta's fins (you can test if its too sharp by gently running a pair of tights (pantyhose I believe is the american term) over the plant to see if it snags or not.
These are my own opinons,Im still a relative newbie at fish keeping but I do source information as much as possible from the forums (as you are yourself with this post) and I am still learning everyday.If I am wrong hopefully someone will correct me.