I created a HUD in a mission and I used safeZone commands to make sure it stays in the same position whatever the user settings.
But if player chooses an interface size bigger than 'Normal' then the HUD is not visible anymore:

• the config

class ZEF_D_INFO_HUD
{
idd = -1;
fadein = 0;
duration = 10e9;
onLoad = "uinamespace setvariable ['ZEF_D_INFO_HUD', _this select 0]";

class Controls
{

		class INFO : RscStructuredText
		{
			idc = 0;
			x = 0;
			y = 0;
			w = 1;
			h = 1;

• the commands that open the HUD

_ZEF_LAYER_HUD_ cutrsc ["ZEF_D_INFO_HUD", "plain"];
uinamespace getvariable "ZEF_D_INFO_HUD" displayctrl 0 ctrlsetposition [safeZoneX + (0 - safeZoneX), safeZoneY + (-0.25 - safeZoneY)];