Check the link for details.
When I'm running my app (that contains the DropDown feature) on my Android Phone, the dropdowns that apears from DropDown menu is really small, even tiny. I tried to set resizeTextForBestFit and resizeTextMinSize/maxSize + change Rect.height of ViewPort (where the dropdowns placed) but it doesn't work. The Scroll Rect on Dropdown List controlling the size. So how can I fix this issue?
Always choose
"Screen Space - Overlay"
and
"Scale with screen size"
It's just one of those weird things about Unity ... 99.9% of the time you use that setting.
In the current version of Unity, Unity screwed-up and set the default wrong.
Only in extremely obscure situations would you use the other settings: it's a huge source of confusion that Unity have the defaults as the wrong settings.
You could have another problem but be sure to do that.
来源：https://stackoverflow.com/questions/38310273/dropdown-gui-resolution-issue-unity-3d