There's are 2 low-tech solutions and 1 high-tech.
First, the easy high-tech solution: if you're licensed for Dynamic Media and using dynamicmedia_scene7 runmode, use Smart Cropping. This is awesome new tech that allows aspect ratios to be adjusted with Sensei. You can configure this under image profiles. Documentation here: https://docs.adobe.com/content/help/en/experience-manager-65/assets/dynamic/image-profiles.html
Low-tech option 1: Only specify one dimension. In your case, you'd specify the width and leave the height blank to let it float. That won't get you the exact dimension you want if the source asset is a different aspect ratio, but without smart cropping, there really aren't good options.
Low-tech option 2: add fit=constrain to the image modifier section. This will instruct Dynamic media to basically do low-tech option #1, but will float either the height or the width depending on which will work better in the specified dimensions.
Hope this helps!
Beau