Setting image size with dynamit tags

Hi, I’ve got a ACF image field on an options page that is set to return an image-array. I use the following dynamic tag to display it in an element block that is hooked in as a mega menu;

{{option key:om-oss_karriar_bild.url|size:portrait}}

Not sure if I use the wrong syntax but the tag only returns the original full image. I’ve tried with the standard image sizes as well, like medium, medium_large. It works when I use the size variabel in query loops for featured images on other places on the site but not here. Are there limitations when using the options page or the ACF image field?

Kind regards, Markus