How should I write thank you letters to members on my thesis comittee?
Instead of a letter, thank them in the 'Acknowledgements' section of your thesis

You can thank your supervisors for guiding you in the course of writing the thesis.

You can thank your examiners for giving helpful feedback that led to improvements in your work (you can do this even if the examiners are anonymous -- it is not unusual for published academic work to thank anonymous peer reviewers for helpful suggestions).

If a specific comment/lead/idea was particularly useful, you can mention it. If one or more supervisors or examiners helped you on the road to publication, mention it.

An example might read like this (feel free to adapt nomenclature to local context and customise with more detail):


  I would like to thank my supervisors, Dr Josephine Bloggs and Dr A.N. Other, both of the Department of Futile Studies, for their support and guidance over several years. I am particularly grateful for their detailed advice and encouragement on how to adapt Chapter 4 of this thesis for publication in The Journal of the Royal Society for the Furtherance of Futile Studies. I would like to thank my examiners, Prof. Isaac Newton and Prof. Immanuel Kant for engaging with my thesis diligently and enthusiastically, providing valuable suggestions: in particular, they helped me clarify my theory on 'The Noumenality of Lincolnshire apples during the plague'. Finally, I would like to thank the lay convenor for my thesis committee, Mr Charles Dickens, senior secretary of the Department of Circumlocution, for his exemplary efficiency in conducting the examination of the thesis.