The TextSize property determines the height in offsets of one line of rendered text. The unit is in offsets, not points (which is used in most document editing programs). It’s worth noting that the “Legacy” font’s line height behaves differently, and won’t match this property exactly.

This property and TextLabel/TextColor3, TextLabel/TextTransparency, TextLabel/TextStrokeColor3 and TextLabel/TextStrokeTransparency each influence the way text is rendered.

This property supersedes TextLabel/FontSize since it is a number and not an enum. Internally, Roblox uses several sets of pre-rendered character images for each size of each font. It chooses the closest size to TextSize, then scales that set of character images to render text. Before the introduction of this property, you could only pick from the pre-rendered sizes, which were listed by the Enum/FontSize enum.