The to-infinitive here stands for "is to", which means "must", in this context.
As for the meaning...Let X = the appliance:
"The inside of X should be between 20 and 50mm deeper than X itself, and should not be more than 550mm in total."
(I don't entirely understand it; it seems to suggest that the inside of the freezer should be 20-50mm deeper than the freezer as a whole, which is clearly impossible. But perhaps there's a technical aspect which I've missed. If so, another member will put me right.)
All the best,
Student or Learner