Ok, well - maybe take a glance at his site and see what the standard size of banners there is - no sense in making one smaller than necessary (or too big

)
A banner should convey as much information as possible in the space provided - assuming you're going to do a static banner (ie - ONE frame of graphics/not flash or rotating GIF) then you should figure out how much space you have to work with and then yes - your logo and tagline would be a good start - as long as they make it clear what you do.
If not - then write up a little sentence about Virtual Assistance services, try to stylize the banner so that it reflects the style of your site and other marketing materials, and add a "click here" - underline that if possible (studies show that people tend to click banners that 'tell them what to do'
Check the ads and banners here on this site and other sites you frequent for more ideas.
Feel free to post your sample if you need help!