Hi @Nitruc
Using firebug I can see that the CSS rule needed to modify the height is:
.acf_wysiwyg iframe {
}
Please note that there is a min-height rule which will need to be set to 0 to allow for your height attribute to work.
Please use firebug / inspector as you can modify the CSS on the fly and test as you go!
Hope that helps.
Thanks
E