How to scale to fit

[JFry]

Jever raised this question recently but I don't want to hijack his post and thought I should post separately.

I too would like my site to fill the screen rather than appear as it does now. The advice given to Jever was to use 'scale to fit'. As suggested by Acorn I have changed the pasteboard colour to white and then I tried to go the 'stretchy full width' route with absolutely no luck at all. For some reason I can't click on the header. I tried to set the stretchy options but not surprisingly I suppose nothing happened.

Would someone be able to give me step by step instructions please? I have attached my xar file. Thank you.