In at least my version of SW (2018 sp4) It is not always possible to edit font size or font type in a table in solidworks. Solidworks needs to basically integrate excel since it's table functionality is so poor, but we will work with what we have for now.

Here is the way to edit font while inside a cell. First click the cell without double clicking (editing) it. Now change the font size to whatever (by clicking the little "A" box in the tooltip that shows when you click the cell), I usually do 9. Now with the Font "A box" still open, double click on the cell. Now you should be able to select text in the cell and change the font or size.

That's it.