I generally buy APC as other brands have been disappointing. As far as size, it depends how long you want to run before shutting down. My power often goes out for just 10 minutes or so, and I have a number of BackUPS 1500 Pro models, but they have many less expensive models and the software will shut down your machine with X minutes left on the battery.
The UPS selector on the APC website will help you select how much battery you need whether you go with APC, Cyberpower, or someone else.