So, that's my question. What are the dimensions of Tolkien's various drawings and illustrations? Does anyone have any guidance on this matter?

Are there specific drawings or maps you are interested in? Tolkien created hundreds of illustrations. If you specified a few, it might be an easier starting point for those with knowledge and expertise to work with and to hopefully provide you with answers.

These sizes can be found in any exhibition catalogs, ie. Drawings by JRR Tolkien (1976), Maker of Middle-earth (2018), Art of the Manuscript (2022).
You can check https://tai.tolkienists.org/ to see if the art in question is included in any of these books.