How this image resizer decides the output size
You give a bounding box and the image is scaled to fit inside it. That is the whole rule, and it is worth stating plainly because "resize" means several different things across different tools.
Fill in only a width and the height follows the original proportions. Fill in both and the image fits inside that rectangle, touching whichever edge it reaches first, which means a portrait photo and a landscape photo given the same box come out at different sizes. Nothing is ever stretched to fill the box exactly. That behaviour, sometimes called "resize to exact dimensions", distorts the picture, and a tool that does it silently is a tool that hands you squashed faces.
If you need an exact frame with no letterboxing, resize first and then crop.
Why it refuses to upscale
Enter a width larger than the original and you get the original size back rather than an enlargement.
This is deliberate. Scaling up interpolates between pixels that already exist, so the result has the same amount of real detail spread over more area, which reads as softness. You end up with a bigger file that looks worse. Every tool that offers upscaling by interpolation is offering you that trade without saying so.
The one exception is SVG. Vector art has no native pixel grid, so rendering it at 2048 pixels is not upscaling, it is just rendering, and the SVG converter on this site lets you pick any size you like.
Sizes worth knowing when you resize image online
A short list covers most of what people come here for. Instagram posts are 1080 pixels square, or 1080 by 1350 for the 4:5 portrait crop. Stories and Reels are 1080 by 1920. YouTube thumbnails are 1280 by 720. Open Graph preview images for link sharing want 1200 by 630.
For a website, the useful habit is to resize to roughly twice the pixel width the image will actually occupy on the page, which covers retina displays without sending a 4000 pixel photo to a 600 pixel slot.
Batch behaviour and local processing
One setting applies to every file in the batch, and each image is scaled against its own aspect ratio, so mixing portrait and landscape in one pass works correctly. Everything happens in this browser tab, so nothing is uploaded and the only limit is your own machine.