I’m not able to delete the Default option – it doesn’t appear in the Manage Devices list for me (I deleted all of the other ones for what I’m about to mention next, so that’s why it’s empty):
What version of Studio are you using (File>About Roblox)? I’m not able to delete Default on version 0.230.0.68937.
How do you fix it if you do accidentally delete it though?
Win+R
regedit > enter
Navigate to HKEY_CURRENT_USER>SOFTWARE>Roblox>RobloxStudio>emulationDevices
Delete emulationDevices in the left pane
Close regedit
Open Studio
All of the emulation sizes will be back