There are many factors that will influence the size of an online advert. Some factors include the objectives of your marketing campaign, where your advert will be published (which advertising network), the type of advert (text vs display ads), which region you chose target, your budget, your targeting criteria and even your customers media habits will influence the sizes.

RECOMMENDED ADVERTS

300px x 250px
Medium Rectangle

300x250

300px x 600px
Large Skyskraper

300 x 600

336px x 250px
Large Rectangle

336 x 280

728px x 90px
Letterboard

728 x 90px letterboard

HORIZONTAL ADVERTS

970px x 90px
Large Letterboard

970x90

468px x 960px
Letterboard

468X60

SQUARE ADVERTS

200px x 200px
Small Square

200x200

250px x 250px
Square

250x250

VERTICLE ADVERTS

120px x 600px
Skyscraper

120x600

120px x 600px
Wide Skyscraper

160x600

MOBILE ADVERTS

200px x 200px
Mobile Small Square

200x200

250px x 250px
Mobile Square

250x250

300px x 250px
Mobile Medium Rectangle

250x250

320px x 50px
Mobile Leaderboard

320x50 mobile