What's stopping you?
You are in the Indesign forum. Document sizes are set in inches, centimeters or millimeters, not in pixels. Why? Because files exported from Indesign are usually intended for printing. For printing, the DPI value (dots per inch, i.e. pixels per inch on screen) is significant. Varying DPI leads to a varying number of pixels in width and height.
For a monitor, means screen, 72dpi is default. For 8x5" banner that'd be 576x360 pixels. However, this is just an example...
Perhaps it'd be better to make the banner in Photoshop?
Well that's not true really.
The default setting is set to Picas and the user can change this to whatever system they are comfortable with.
I use Millimeters, not inches.
And you can create a new Document and choose "Web" and you are given Pixels by defualt.
And a monitor isn't necesarrily 72dppi - it's really 96ppi - and that's what Acrobat is set to by defualt. Web images are 72 ppi for purposing for the internet - but 72ppi images look fuzzy in Powerpoint as the monitor is really 96ppi (circa).
And the best place to make the Banner would actually be Fireworks.