Let's assume your application running in HTML5 client requires more size in height than default height value reported by your browser. You have 4 different ways how you may proceed.

A. Full-screen button. Before starting your application
    1. click on Top-menu and go to Full-screen by clicking on Full-screen button
    2. start your application
This would make your RDP area screen to be always equal to the size of your Full-screen area and rely on the actual size of your monitor. That will work only on Windows/Linux PCs, or with Mac when used Mac-Chrome/Mac-FireFox, but that won't work for mobile devices. More about Full-screen usage can be found here: How to use full screen mode in HTML5 client?


B. Screen size hot-change. This way may meet your needs more effectively but could potentially make your screen looking uglified since the screen proportion ratio would be not 1:1 anymore and browser would try to squeeze the image to fit into browsers area. This way should work on all PC/Mac browsers similary. Before starting your application
    1. on HTML5 page inside top address bar where you see *****/html5.html click there with the left mouse, 
        so that it gets focus
    2. now press buttons "Ctrl" + "-" so oft you get satisfied screen size. 
        (on old Windows systems that do not support on-the-fly-resizing just refresh the tab after any such change)
    3. start your application
(on older Internet Explorer 11 it is not allowed to call Ctrl+- when focusing address bar, there you have to do that on portal page by focusing HTML page area self and only after that start new session which would take the new dimension. (IE Edgeunaffected)


C. Mobile-browsers only. On mobile browsers it works differently than on PC browsers. Since your program is very specific and requires minimum height rather than just width then do following 
    1. locate file on your server *\Clients\www\software\html5\settings.js
    2. open it with Notepad
    3. locate inside variable W.viewportminheight = "200";
    4. and change its default value, as example to 1400 like that > W.viewportminheight = "1400";
    5. now start new session and then your application.
In such case when ever your mobile browser will report size lesser than 1400pixels, it will enforce at least 1400px for RDP area, but make overflowing area to be scrollable down/up by panning (moving up/down with one finger).
More about changing viewport parameters can be found here: Change default viewport width for mobile devices.


D. Third part program support contact. These actions should be done by you anyway if possible since not TSplus related. 
     1. contact creators/developers of your specific program 
     2. report your minimum screen size problem as a bug 
     3. additionally ask to make that area either scrollable or auto-adjustable according to the actual screen size.
However self-evident we can't assist you on that point!