The new Marineland Stealth Visi-Therm models are short and stout as opposed to long and thin (like the Marineland Stealths of the past) so they fit nicely in smaller tanks.
The Hydor theo 25 watt is REALY compact so I'd say it would fit into whatever you have.
Then again the shape of your container (Bowl, Jar, Vase, long and short, narrow and tall, round like a bowl and short or tall) all come into play when choosing a heater.
I find 1 gallon jars and vases are more heater-friendly than bowls for the most part. =] Especially if you want a quality adjustable heater. =]
if you get a 25 watt adjustable heater you could continue to use it in a 5 gallon as well. =]
I highly recommend the Hydor Theo and the Marineland Stealth Visi-therm. =]
Also it's difficult to find nice heaters for nano tanks in stores. :[ I'd go with buying from Big Al's petstore online if you can! =]
They're having a sale on the Hydor Theo and lemme tell you this sale is AMAZING! <3 It's nearly half price! Everywhere else you look online 25 watt heaters will be about $20.00!
Another great aspect to ordering online is that things tend to be cheaper (especially heaters) than they are in-stores.
Both of my Marineland Stealth heaters cost me about $35.00 in store. :[ I got mine from Petsmart too and that's typically cheaper than PetCo. :/
So your best bet is buying them online! =]
If you're younger then I'd show your parent/guardian all of the different store prices like Petsmart and Petco VS Dr. Fosters and Smith Aquatics and Big Al's and once they see Big Al's sale they'll most likely order it for you.
If you're of age and have the cash to order it yourself or have a friend with a credit card or paypal that'll do it for you then I say go for it! =]