Jever, you are using a very recent XDA (Xara HTML filter v.9.6.2.48) so the following should help you:

Object Selection: Page & Layer gallery > Layer View: double-click a thumbnail then toggle Unlock icon on dark grey top-right > (un)locks everything.
(Object) Stretch to Full Width: Selected object > Utilities > Web Sticky/Stretchy > Stretch properties: Full width.
(Website) Scale to Fit Width: Utilities > Export > Scale to fit: Scale to fit width: maximum 1280 (my recommendation).

Acorn