Use a 32-bit targa or tif file for the export.
One alternative or supplemental method you might try is to export/save the frame as a .pdf file, only because scaling up a PDF can appear sharper sometimes, since you’re working in vector, rather than raster.
Before you export the still, do everything you can in the video timeline to color-correct and enhance it right there, first. Save in the highest resolution you have available. Then take the still into PhotoShop or other image editing program, stack a couple of identical layers, and play with adjusting blending modes and opacity between layers, as you manipulate color and shading on those layers. You can’t add resolution, but you can affect the *apparent* resolution by sharpening the contrast and extending the dynamic range.