If you include only banners in one or two sizes with your campaigns, you will miss opportunities to serve banners on sites that use other ad formats.
SharpSpring Ads accepts banners in the following sizes: 300x250, 728x90, 160x600, 300x600, 468x60, 120x600, 970x250, 970x90, 320x100, or 336x280
The 300x250 and 728x90 are by far the most common ad formats. If you're only making two banners for a campaign, make them in those sizes. But leaving out the other sizes will reduce the reach of your campaign as your ads won't serve in ad positions in the other size formats.
To see the dimensions of the banner ads you've uploaded, go to your Manage-> Ads page and look at the information in the ads table.