Thanks for the suggestion, but this is not a "quick and easy" way if I have e.g. 50 pictures imported into the document that have to be made of equal size before arranging them into a figure for publication (something that I have to do routinely for my work). I will have to type 50 times the required values for each individual picture.
Some graphics programs have a "Make equal size" command that can achieve this for the whole batch in one or two clicks, but such a command seems to be missing in Illustrator. The "Transform each" command does not allow to specify a size and the scaling option is useless in my case.
Assuming that each of your graphics exists somewhere as an individual file, I'd do this project in InDesign. Make a grid of box frames of the size you want, apply Place and get all the graphics at once, and click once on each box in order. Drag select all the boxes and apply Object - Fitting - Fit Content Proportionately.