In Size properties there is the relative size and the pixel absolute size. By default they always come with pixel absolute size. In this case you want to be using relative.
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The image above has a 100x100 pixel size set. Size uses a UDim2 value to set the value. And UDim2 value looks like this:
({ScalarX, PixelX}, {ScalarY, PixelY})
Basically change the size using ScalarX and ScalarY to keep it responsive.
You may know more about Sizes right here:
https://developer.roblox.com/en-us/api-reference/property/GuiObject/Size
And this is a how to guide to responsiveness in Roblox: