In my copy editing work I often need to resize figures to make them fit the pagewidth (remove "Overfull \hbox..."). I always try to keep the author's layout and the proportion between the figures.
Here's a MWE:
\documentclass[11pt]{article} \usepackage{amsmath} \pagestyle{empty} \usepackage{graphicx} \usepackage{subfigure} \usepackage{blindtext} \overfullrule=1mm \begin{document} \begin{figure} \subfigure[]{\includegraphics[scale=.55]{example-image-a}}\subfigure[]{\includegraphics[scale=.6]{example-image-b}} \caption{Image.} \end{figure} \blindtext \end{document} Below I'll post a solution that I figured out to quickly handle these cases. I'm open to other tips and solutions.
Clarification. I begin to suspect that my question isn't clear enough (maybe for my poor english). My solution is focused in keeping the proportions between all the images' size (not the ratio of heigth and width in the single image). I need to scale all figures by the same value as fast as possible.
In the absence of "subfigure" my solution, in this particular case, would have been:
\begin{figure} \resizebox*{1\textwidth}{!}{\includegraphics[scale=.55]{example-image-a}}\subfigure[]{\includegraphics[scale=.6]{example-image-b}} \caption{Image.} \end{figure} 

widthoption instead of thescaleoption? For two side by side images you wan chose widths relative to the textwidth. Both widths should add up to q value smaller than or equal to the textwidth.widthkeeping the original proportion of the figures may be very tricky/boring.widthoption, the aspect ratio of the image stays unchanged.