This depends on what each theme provides.
- Check which of them you are using.
- Then look for its index file in
/usr/share/icons
Example: /usr/share/icons/elementary-xfce/index.theme
[Icon Theme]
Name=elementary Xfce
Comment=Smooth modern theme for Xfce
Inherits=adwaita,gnome,hicolor
Example=directory-x-normal
#Directory list
Directories=actions/16,actions/22,actions/24,actions/32,actions/48,actions/64,actions/96,actions/128,actions/symbolic,animations/16,anim
ations/22,animations/24,animations/32,animations/48,animations/64,animations/128,apps/16,apps/22,apps/24,apps/32,apps/48,apps/64,apps/96
,apps/128,apps/symbolic,categories/16,categories/24,categories/32,categories/48,categories/64,categories/96,categories/128,categories/sy
mbolic,places/16,places/22,places/24,places/32,places/48,places/64,places/96,places/128,places/symbolic,mimes/16,mimes/24,mimes/32,mimes
/48,mimes/64,mimes/96,mimes/128,mimes/symbolic,devices/16,devices/22,devices/24,devices/32,devices/48,devices/64,devices/96,devices/128,
devices/symbolic,emblems/10,emblems/16,emblems/22,emblems/24,emblems/32,emblems/48,emblems/64,emblems/128,emblems/symbolic,emotes/16,sta
tus/16,status/22,status/24,status/32,status/48,status/64,status/128,status/symbolic,notifications/16,notifications/22,notifications/24,n
otifications/32,notifications/48,panel/16,panel/22,panel/24,panel/48,stock/16,stock/22,stock/24,stock/32,stock/48,tools/22,
[actions/16]
Size=16
Context=Actions
Type=Fixed
[actions/22]
Size=22
Context=Actions
Type=Fixed
[actions/24]
Size=24
Context=Actions
Type=Fixed
[actions/32]
Size=32
Context=Actions
Type=Fixed
[actions/48]
Size=48
Context=Actions
Type=Scalable
[actions/64]
Size=64
Context=Actions
Type=Scalable
[actions/96]
Size=96
Context=Actions
Type=Scalable
[actions/128]
Size=128
Context=Actions
Type=Scalable
[actions/symbolic]
Size=16
Context=Actions
MinSize=8
MaxSize=512
Type=Scalable
...
Keep in mind, that it Inherits=adwaita,gnome,hicolor
, so parent themes may have additional sizes. Non-standard sizes could be covered by Type=Scalable
icons.