A lot is going to depend on the freezer and its performance. Is it a machine you already have?
If so, I would connect it via one of those plug in power monitor devices and measure what it uses over 48 hours. That will give you some idea of storage capacity you need.
Next measure the operating current, BUT beware, motor fridge compressors have a very large switch on surge which would probably cause an inadequate UPS to shut down rather than start it.
Then, as Murdoch recommends approach a UPS manufacturer or specialist supplier.