How to adjust the Font Size of tableGrob

You can do this via themes:

mytheme <- gridExtra::ttheme_default(
    core = list(fg_params=list(cex = 2.0)),
    colhead = list(fg_params=list(cex = 1.0)),
    rowhead = list(fg_params=list(cex = 1.0)))

myt <- gridExtra::tableGrob(mtcars[1:5, 1:5], theme = mytheme)

grid.draw(myt)

There are a number of other examples in browseVignettes("gridExtra") -- look at the tableGrob examples. A great deal of control is possible.

You can also change the general font size easily with base_size

grid.draw(tableGrob(head(mtcars, n=10),theme=ttheme_minimal(base_size = 5) ))