You can go with a larger banner resolution for LPs as you don't have to worry about filesize/dimensional restrictions that email clients impose. If simply working with a LP using absolute values, you can go with something like 960px which is fairly standard and should look acceptable in most resolutions.
If your LP is coded more responsively however, you can afford to aim higher, safe in the knowledge that your web-code will always ensure a good fit for the user's screen resolution. Here's a good article on responsive web design
Whichever way you go about implementing your banner, always test thoroughly on multiple devices. Consider testing a couple different image sizes to compare which looks best overall.
Hope that helps in some way!