If you put the icon to a texture control (controls:
https://wiki.esoui.com/Controls) of type CT_TEXTURE you just need to change the size of the control via
textureControl:SetDimension(x, y)
or
textureControl:SetWidth(number)
textureControl:SetHeight(number)
Controls get anchored to other controls so make sure the anchors are removed
before resizing, as else the anchors could keep it "not resized" as they grab left/right at other controls to stretch it.
textureControl:ClearAnchors()
Afterwards you can resize and reposition it and then re-anchor it f needed via
textureControl:SetAnchor(parameters here)
->
https://wiki.esoui.com/Control:SetAnchor
If you do not use any texture control but only text output of a texture, within a String, you can resize them too via ZOs API functions:
https://wiki.esoui.com/Text_Formatting#Texture