I really doubt you want to put a true longwire (which is at least 1 wavelength long at a specific frequency) on a portable - I'd recommend a very simple Inverted L like this one...
(you can safely ignore the matching network at the bottom of the pic)
Personally speaking, I probably wouldn't go any longer than 45-50 foot total length. And if you are using the antenna jack - and you should - read the caution on the Universal website, the link for which is in blue.
And of course the first timer project is highly recommended...