Marker size

For a map I’m building, I am uploading custom markers (flag images), which I connect with locations. This works fine, but I’m having an issue with image sizes. Especially on retina screens, the small markers don’t look sharp.

Is there a way to define with marker size? It would be useful to be able to define this, then I could set that a market should show at 24px size, but I can upload a 48px (or larger) image and get it to show it in better resolution.

I have tried using CSS such as:

.wpgmp_map div div div div div div img[src*="uploads"] {
width: 20px !important;
height: 20px !important;
}

which allows me to target marker images from the uploads folder (and not any general map images), but because the plugin places the markers with position absolute and this gets done first, changing the marker size with CSS breaks the positioning.