You can set anchors instead of width/height.
In lua:
frame:ClearAllPoints() -- removes all anchors
frame:SetPoint("TOPLEFT","UIParent","TOPLEFT")
frame:SetPoint("BOTTOMRIGHT","UIParent","BOTTOMRIGHT")
in xml:
<Anchors>
<Anchor point="TOPLEFT" relativeTo="UIParent" relativePoint="TOPLEFT"/>
<Anchor point="BOTTOMRIGHT" relativeTo="UIParent" relativePoint="BOTTOMRIGHT"/>
</Anchors>
This will stretch the frame to cover the whole screen even if it resizes. I commonly use this to stretch a background texture in a frame. Once anchored to both corners there's no need to go back to it if I change the size of the window.