There should be an option to make one or more controls the same size as another selected control on the form. This is a very common option to have in graphical interface designers, as seen in Netbeans and Visual Studio.
Select a control: then select X number of controls, right click to get options to "Make controls the same size" >> first selected control is the control that determines the size of all other selected controls.
As we are currently focused on Universal Forms, this is not on the short to medium term.
Once we have shipped Universal Forms for SharePoint, we will revisit Classic Forms.