By default, PopupMenus/ComboBoxes are dependent on the size of the font but the size of them should be able to be changed with
SendNotifyMessageW(PopupMenu1.Handle, CB_SETITEMHEIGHT, -1, 50) which sets the height of the base control to 50
It looks to be a bug that its not taking the size from the Height parameter in the IDE.
From an initial test, Xojo are using the correct system to resize the control, but they are doing it at the wrong time. It seems like they are setting the height when the control is clicked (but only to the default height) and not when its opened.
If you place the following code into the Popup.Open
Const CB_SETITEMHEIGHT = &h0153
Declare Function SendMessageW Lib "User32" (hWnd As Integer, Msg As UInt32, wParam As UInteger, lParam As Integer) As Integer
Call SendMessageW(PopupMenu1.Handle, CB_SETITEMHEIGHT, -1, 50)
You see that it correctly sets the height to 50. As soon you click on the control it changes height again (bug?), I assume that Xojo is doing something behind the scenes here when it shouldn't be, probably firing another SendMessageW at the control which should actually be firing at the open, and not the click.
You can see this happening if you copy the above code into PopupMenu.Open and PopupMenu.MouseDown, when you click the control a few times and the initial popup has rendered and closed, it then works for all future updates.
Michel would have to chip in here to let us know how he got around that, I could probably find out with a bit of trial and error but it would be quicker if he wants to chip in, I dont know if he wants to as he might want to protect the IP in his control.