The Postgraduate Degree Regulations stipulate the word count for research degrees.
The thesis must not exceed a maximum word count of 100,000. There is no minimum word count. The word count of the thesis includes the main text, preface material, footnotes and references but does not include material in the appendices, bibliography, abstract or lay summary. In exceptional circumstances, on the recommendation of the supervisor, permission may be granted by the College to exceed the stated length on the ground that such extension is required for adequate treatment of the thesis topic.
More information on what counts towards the word length is available here.