This is something I figured out last year how to do, forgot, then struggled to figure out how I did it again! If I had blogged about it, I'd have known! Instead, the SDN Forum was kind enough to remind me.
As a Custom Field Type for this particular aspect of my web page was too cumbersome to develop within my deadline, I opted for an IFRAME Field Type, which when given a URL simply exposes a web page via which you can edit the relavant page data. Unfortunately, the default height is a miniscule 80 pixels. You can override this height, however, by:
Going to Template Manager
Opening your Template and selecting Standard Fields on the View ribbon:
Expanding the individual field tree on the left and selecting the relavant field
FInding the "style" field in the Appearance section and entering your own CSS, in my case "height: 200px".
Then, when you navigate to the Item which implements the Template, you get a nicely increased height:
I should add that you must remember to deselect Standard Fields once you are done, otherwise Saving subsequent content will override any shared fields and therefore break the link between Template > Master > Item.