The monitor size isn't very important but the resolution is.
My monitor's resolution is 1920x1080 but I always make sure all the forms can be fitted into 800x600 because most monitors can support the resolution. I use anchors, align and panels to make it 'looks good' on larger resolutions. Sometimes, the form's size should be set to auto maximized.
For example buttons that should be always on the bottom of the form, I set its Align := [akLeft, akBottom]; For the objects that can be resized, like grids and memos, I set their Align := [akTop, akLeft, akRight, akBottom]; For some cases, putting the objects into a panel will make the positioning easier.
And yes, TFlowPanel, TForm.Scaled and LCL Scaling can be useful. But I never use them, not much I can say about them.