Hi
A 2.5 gal or 5 gal works great. If you don't want the trouble of getting a filter and light, you can always purchase a small hex or triangle under gravel tanks. Pet Smart and Walmart have them.
Depending on the temp of the office place, you may want to add a small heater. The Hydor mini works well and fits in a small tank.
With the undergravel filter, you just need to change 15-20 % of the water once a week. Use a turkey baster, less messy.
I have the majority of my male Bettas in these tanks. The girls are in 10-15 gal tanks.
Don't over feed your Betta and add a few plants and he will be fine.
I like to add a 1 1/2" PVC pipe connector, for them to hide in. My guys love them. Cost about 0.59 at the hardwear store.
Bettas are ok if you miss a feeding or two, just don't go much longer. Over a weekend without food is OK.
I am the President of Betta Breeders Canada, we are affiliated with the International Betta Congress.
We love people who give their Bettas room to move and lots of attection.
Good luck.
Catherine