Before you export, you need to understand the link between size, resolution and the final use.

Two distinct ideas

• Pixel dimensions — the number of dots that make up the image (e.g. 1920 × 1080).
• Resolution (dpi) — the pixel density, which is decisive for print.

The right value for the job

• Screen / web: 72 dpi is plenty, and you think in pixels.
• Print: 300 dpi for a sharp result.

Go to Image → Image Size to adjust it. Careful: enlarging a small image makes it blurry.

Key point: you can scale an image down without loss, but never up without degrading it. Always start from an original that’s big enough.