I've been looking into getting a Betta and was wondering if this would be acceptable. I have an 11L tank (3 US Gallons/2.5 Imperial Gallons) and was wondering if that would be an adequate size for a single Betta. I have a Small World filter, but I've read that Bettas prefer a tank without a filter? I also have a 25W heater from a friend, but I was wondering if 25W is too powerful for a tank of this size?
Thanks if advice, if you know of any other things a noob Betta owner should know then feel free to tell me!
Thanks if advice, if you know of any other things a noob Betta owner should know then feel free to tell me!

Sponge filters are ideal for bettas. They don't like a fast flow current. 
/www.fishforums.net/index.php?/topic/113861-fishless-cycling/ recommends having it higher than when you have fish, around the mid to upper 80s F. The filter will cycle without a heater but it will take longer.