Resizing attribute form in QGIS 3

Open Layer properties - Attributes form - switch from Autogenerate to Drag and drop designer (worked for me).

Layer properties Attributes Form tab

This is a known bug:

It's Mac-specific, and may be related to a similar issue with the Identify results window. It doesn't look like anyone is working on a fix at the moment. See the issue tracker page on github for more details and up-to-date information.

github user emonshaugen suggested this workaround:

When you create a new feature, and the Attribute Form pop-up appears, you can hit the green (+) symbol to maximize the dialogue box. From then on (until you shut your project down) the dialogue box will pop-up full screen.