Standard banner size

Name	Width / px	Height / px	Aspect ratio
Rectangles and Pop-Ups
Medium Rectangle	300	250	1.2
Square Pop-Up	250	250	1
Vertical Rectangle	240	400	1.67
Large Rectangle	336	280	1.2
Rectangle	180	150	1.2
3:1 Rectangle	300	100	3
Pop-Under	720	300	2.4
Banners and Buttons
Full banner	468	60	7.8
Half banner	234	60	3.9
Micro bar	88	31	2.84
Button 1	120	90	1.33
Button 2	120	60	2
Vertical banner	120	240	2
Square button	125	125	1
Leaderboard	728	90	8.09
Skyscrapers
Wide skyscraper	160	600	3.75
Skyscraper	120	600	5
Half page ad	300	600	2

Sizes in bold are part of the IAB's Universal Ad Package.

resource: http://en.wikipedia.org/wiki/Web_banner

DESIGN RESOURCES

Standard Web Banners

Size	Style	Gif Weight	Flash Weight
468 x 60	Full Banner	20 KB	30 KB
728 x 90	Leaderboard	25 KB	35 KB
336 x 280	Square	25 KB	35 KB
300 x 250	Square	25 KB	35 KB
250 x 250	Square	25 KB	35 KB
160 x 600	Skyscraper	20 KB	30 KB
120 x 600	Skyscraper	20 KB	30 KB
120 x 240	Small Skyscraper	20 KB	30 KB
240 x 400	Fat Skyscraper	25 KB	35 KB
234 x 60	Half Banner	15 KB	20 KB
180 x 150	Rectangle	15 KB	20 KB
125 x 125	Square Button	15 KB	20 KB
120 x 90	Button	10 KB	20 KB
120 x 60	Button	10 KB	20 KB
88 x 31	Button	10 KB	10 KB

Common, But Not Standard Banner Sizes

Size	Style	Gif Weight	Flash Weight	Example
120 x 30	Button	10 KB	10 KB
230 x 33	Small Banner	15 KB	15 KB
728 x 210	Large Leaderboard	30 KB	40 KB
720 x 300	Large Leaderboard	25 KB	35 KB
500 x 350	Pop-up	30 KB	40 KB
550 x 480	Pop-up	30 KB	40 KB
300 x 600	Half Page Banner	35 KB	45 KB
94 x 15	Blog Button	10 KB	10 KB

measurements are in pixels.
Check with the website where you plan to place the banner ad for their specifications; their specfications may be different. Designers Toolbox is not responsible for errors.

resource: http://designerstoolbox.com/designresources/banners/