It looks like you’re using Offset to set the size of the text. You’ll need to switch to Scale.
In a UDim2, there are two different size types: Offset and Scale.
Offset is a specific amount of pixels.
Scale is a float determining the size based on screen size.
Your .Size property may appear like this in the properties window: {0, 100},{0, 200}
The separate {}s are the X and Y axes, and the two numbers inside each of them are the Offset and Scale values.
In this case, 0, 100 is the size of the object on the X-axis. The 0 is the scale, and the 100 is the offset.
An easy way to speed up the process of switching from offset to scale (and vice-versa) is using this plugin.