But of course what works in general may not work for your journal! So there's one set of options that's journal-specific.
If you're mostly happy with how things appear, but want to tweak the width at which changes are applied to your journal, go to your journal customize settings > Additional Options, and then look for
"The width at which the layout switches from single column mode (mobile view) to multiple columns if selected (tablet view). "
"The width at which the layout switches to larger screen mode (desktop view) and can afford some extra whitespace. "
Setting a width in the first option will make the biggest difference; the second option should only be minor whitespace tweaks depending on the layout.
Meanwhile, if what you want is to control how you view journals across all of Dreamwidth, there's a different option for that. If you'd like to always view the desktop / big screen version on your mobile phone or tablet, go to your Account Display Settings and check "Turn off mobile view for this device".
That will carry over to all journals you view whether you're logged in or logged out. It's device-specific* so if things work on your phone but not your tablet, or vice versa, you can check or uncheck this setting per-device.
* more accurately, it's browser-specific. If you're using multiple browsers on a device, you'll have to check it for each browser.