Size determines how big objects are in Roblox. They affect things from bricks to GUIs to meshes.
2 Types of Size[]
The explorer and properties toolbar of a GUI TextButton. Size is highlighted in green on the properties toolbar.
Size applies to both 2D and 3D objects in Roblox. They are both listed below.
2D Objects[]
How to Sizing it Works[]
GUIs use something that looks like this:{0,85},{0, 29} for sizing. The numbers in the left set of brackets stand for it's horizontal (left and right) size and the right set stand for it's vertical size (up and down). The left number in each bracket stands for it's size compared to its parent object. For instance, if it's inside of a GUI frame and you put 0 in the left column of each it will be, well, nothing! if you put 1 on the other hand it will be the exactly the same size as it's parent. But, if you put in 0.5 it's height will be exactly half it's parent's height and width. The second number in each bracket does the same thing, but does it by the amount of pixels you want it to be, having nothing to do with other objects.
Objects it Affects[]
These are the 2D objects that are affected by size:
3D Objects[]
How Sizing it Works[]
3D objects use a simpler way of sizing than GUIs. They look something like this: (512, 0.4, 512). The first number is the length, the second is the height, and the third is the width.
Objects it Affects[]
Size effects any BasePart. It also effects the size of any GuiObject.