I'm afraid that is rather general.

London rooms may be smaller than you are used to, but I don't know what you expect! It is possible that people complain about room size because they have unrealistic expectations.

However, at some of the less pleasant properties then a room may have as many beds as will physically fit with just about no other space. You can't always tell by price which these properties are.

I would take the quality of the hotel as a better rule than the number of beds, but even then it is worth looking for reviews.

I'm not so sure you need to be near Victoria, it depends where you are visiting - you will only be coming/going to the airport once. There are also direct trains to London Bridge if that suits you better.

(If you have hotel points to use, London may be a good place for it.)