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After launching your game, go to "User Settings" > "Client Size" and choose your preferred size.
The range is between 50% and 300% in 5% increments.
After changing size for the first time you may find it helpful to close and re-open the client to ensure that your activity log and chat text are correctly in proportion with the rest of the client, this is optional and likely only matters if you've made a large increase / decrease in size.
In the unlikely event you're looking for the old advice around using Windows magnifier (or other options) a copy of that can be found here for enlarging and here for smaller screen issues.