Is the logo for each organisation restricted to a certain size on the website or is there a way to increase its dimensions so that it becomes more prominent? And does this vary between the different styles: Bootstrap, Modern or Fresh?
The size is different in each theme yeah. The main Voice site is now using the Fresh theme and you'll see that it has a pretty big logo.
If you upload a good sized logo into the site settings, then the system will resize it as appropriate depending on your chosen theme.