There's a reference about the Human Interface library (and all the EasyPLC external libraries) to see it you must select any Phase or block that accepts the script language, then on the lower tool tree click in the PLC Script Reference, the a new window appears showing all the script functions/procedures available and the information about the parameters/returned values.
I'm going to send you a video explaining this procedure, this video is a exe file, that allows more compression ratio.
The sizes are Twips, this is a measuring unit used by Visual Basic, the Human Interface library is written in this language for this reason are used the same units. A very useful trick is to use Visual Basic 6.0 to design your window, once is finished copy the values of the elements placed on the form in your script code, then you earn a lot of time. Also you must to know that all the Human interface elements available (command buttons, picture, labels, text boxes, etc...) have the same properties that the Visual Basic controls.
Regards.