As usual I see this from a different viewpoint.
You have a 200x200 image with text. You have a pattern that is 500x500 and want to fill the background of the text. The fill tool will fill 200x200
My take on it.
Open the 500x500 pattern as an image.
Copy and paste-as-a-new layer under the text layer. Move this layer around with the text tool.
When positioned, Layer -> Layer to image size crops 500x500 to 200x200
like this ? https://i.imgur.com/79ZmIAs.mp4
You have a 200x200 image with text. You have a pattern that is 500x500 and want to fill the background of the text. The fill tool will fill 200x200
My take on it.
Open the 500x500 pattern as an image.
Copy and paste-as-a-new layer under the text layer. Move this layer around with the text tool.
When positioned, Layer -> Layer to image size crops 500x500 to 200x200
like this ? https://i.imgur.com/79ZmIAs.mp4