You might find this field of view calculator handy...
https://www.scantips.com/lights/fieldofview.htmlYou can enter focal length, distance, and sensor size and it will show you the dimensions of the subject area covered with those specified parameters.
The "field distance unit" you enter can be considered as anything you want it to be (inches, feet, meters, etc.) and the "field dimension" displayed will reflect that same unit of measure. The math formula is always the same regardless of the unit of measure, so it isn't specified as anything other than a "unit".
For instance a 63 mm lens at 8 feet (using feet as our distance "unit") with a 33 x 44 mm sensor would cover an area of 4.1905 feet by 5.5873 feet (using the same distance "unit") or in inches 50.3" x 67" ("field dimension" in feet x 12 for inches).
Hope it helps.
As an additional note... I would avoid using lenses that are shorter than a moderate wide-angle focal length. The 45 mm should be fine, but I would be hesitant to use anything much shorter in order to avoid issues caused by the distance from the edges of the painting to the lens being much greater than the distance from the center of the painting to the lens causing potential distortion or other uniformity issues.