My experience is…
#_LOCATIONMAP in the format settings of a single event cannot be controlled by CSS. Whatever you add to CSS….it has no impact to the size of the map. If one puts e.g. width:300px in CSS, the map will be shifted but not resized!
The only way to adjust the size of the map is using the MAP-Settings EM -> Settings -> FORMATS -> MAP -> Width and Height.
Contrary to #_LOCATIONMAP the map showing all event locations can be controlled by CSS.
I have done a lot of testing (disabled plugins, changing themes, removing tweaks in functions.php, adding all kind of CSS….) – no chance!