Reshade Image Enlarger is a once commercial, now free tool which aims to enlarge your images up to 2000% without the usual blurring and jagged edges.
Open any image, set a new width and/ or height, Reshade Image Enlarger analyses the picture and upscales it with optimal settings. You're then able to preview the results in areas of key detail (faces, edges, wherever you like).
If you just want to see how the image might look at different resolutions, holding down Ctrl while spinning the mouse wheel will increase or decrease its size, updating the preview a few seconds later. You could use this to focus on a face, for example, then increase its size as much as possible while maintaining acceptable quality.
Reshade Image Enlarger tries to find the best possible settings for each image automatically, but you can also fine-tune the results by dragging various sliders (Denoise, Reduce Artifacts, Accuracy, Texture).
An "Advanced Options" tab provides even more tweaks. If your image seems overly "smoothed", for instance, try increasing the "Smooth Gradients" value.
Once you've found the best settings you're able to save the current image, or drag and drop multiple folders and images and apply the settings to them all.
We resized a few test images with Reshade Image Enlarger and Paint.Net, compared the results, and found that Reshade's versions had consistently sharper and more clearly-defined edges.
There can still be problems. Areas with a lot of closely-packed fine detail sometimes appeared overly smoothed, for example.
Reshade Image Enlarger does more than the usual Resize function, then, but it can't work magic, the differences aren't always huge and sometimes they might be worse. Our best advice: just try it, see what the program can do for you.