The common screen resolution by most devices, especially mobile phones is 320x480, but that is obviously not sufficient to create the RDP session.
Therefore the resolution was preset to 800 in width. The height of resolution gets recomputed by hidden browsers native logic. The higher the width the bigger the height.
The height and width depend on landscape/portrait view of your device, like 800x904 or 904x800 etc. Each browser can recompute it on its own logic to fit the rdp screen into the viewport of your device and may differ depending on the browser even when used on same device.
If you set the height manually, then you will break the viewport ratio of your device and the final RDP session will be out of your port view, and to reach these areas you will have to scroll to wished positions.
If you need more height, increase the width!
By testing on mobile phone devices, the good value for width was 800. Though you must pay attention : the CPU's on most mobile phones are usually slow, therefore when you increase the height, it will increase the CPU load. On tablet devices the CPU's are faster, therefore it is recommended to set the width to higher value like 1280 and allow the device to recompute the value for height.
Because some browsers like FireFox mobile do not allow the setting of viewport after page loading, this value was set fixed into the Clients\www\software\html5.html file:
<meta name="viewport" content="width=800, maximum-scale=1.4">
For example change it to:
<meta name="viewport" content="width=1280, maximum-scale=1.4">
to increase width and at same time height recomputed by browsers native internal logic.
As a second example, changing it to :
<meta name="viewport" content="width=1280, height=1400, maximum-scale=1.4">
would break viewport area and RDP session would not fit the screen.