Pixel dimensions describe the grid
A 1600 by 900 image contains 1600 pixels across and 900 down. The ratio is 16:9; resizing one side without adjusting the other changes that ratio.
Pixel dimensions do not by themselves specify how large an image will print. Physical print size also depends on output resolution and the workflow receiving the file.
Keep the ratio when fitting a target box
With the ratio locked, changing width or height derives the other dimension from the original proportions. Use the lock when you are fitting a photo or illustration into a known width.
Turn the lock off only when stretching is intentional, such as preparing a texture. Check faces, circles, and text for distortion.
Avoid enlarging small sources for detail
Upscaling interpolates new pixels between existing ones. It can make a file meet a dimension requirement, but it does not recover fine detail absent from the original.
If a destination needs more pixels, look for a higher-resolution original or an appropriate vector source rather than enlarging a small raster blindly.
Common questions
What does the enlargement option do?
It allows the canvas to draw the source into a larger pixel grid. It does not add captured detail.
Can a smaller image look sharper?
Sometimes a smaller output looks cleaner at its display size because resampling reduces high-frequency detail, but inspect the actual subject and text.
References
Sources explain browser behavior or guidance; they do not verify a particular output file.