Using this VB.net code :
_log.DebugFormat("Request.UserAgent : {0}", Request.UserAgent)
' .net
_log.DebugFormat(".net Request.Browser => Height : {0} / Width : {1}", Request.Browser.ScreenPixelsHeight, Request.Browser.ScreenPixelsWidth)
' 51degree.mobi
Dim provider As Provider = Factory.ActiveProvider
Dim device As BaseDeviceInfo = provider.GetDeviceInfo(Request.UserAgent)
Dim d = device.GetAllProperties()
_log.DebugFormat("51degree.mobi => Height : {0} / Width : {1}", d("ScreenPixelsHeight")(0), d("ScreenPixelsWidth")(0))
' Telerik
Dim screenDimensions As DeviceScreenDimensions = Detector.GetScreenDimensions(Request.UserAgent)
_log.DebugFormat("Telerik => Height : {0} / Width : {1}", screenDimensions.Height, screenDimensions.Width)


I got this results :

Navigateur Galaxy S4
Request.UserAgent : Mozilla/5.0 (Linux; Android 4.3; fr-fr; SAMSUNG GT-I9505 Build/JSS15J) AppleWebKit/537.36 (KHTML, like Gecko) Version/1.5 Chrome/28.0.1500.94 Mobile Safari/537.36
.net Request.Browser => Height : 800 / Width : 480
51degree.mobi => Height : 800 / Width : 480
Telerik => Height : 640 / Width : 360

Navigateur Galaxy S4 (émulateur)
Request.UserAgent : Mozilla/5.0 (Linux; U; Android 4.3; en-us; Samsung Galaxy S4 - 4.3 - API 18 - 1080x1920 Build/JLS36G) AppleWebKit/534.30 (KHTML, like Gecko) Version/4.0 Mobile Safari/534.30
.net Request.Browser => Height : 1024 / Width : 600
51degree.mobi => Height : 1024 / Width : 600
Telerik => Height : 0 / Width : 0

Chrome sur Galaxy S4
Request.UserAgent : Mozilla/5.0 (Linux; Android 4.3; GT-I9505 Build/JSS15J) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/34.0.1847.114 Mobile Safari/537.36
.net Request.Browser => Height : 1280 / Width : 768
51degree.mobi => Height : 1280 / Width : 768
Telerik => Height : 640 / Width : 360

Iphone
Request.UserAgent : Mozilla/5.0 (iPhone; CPU iPhone OS 6_1_3 like Mac OS X) AppleWebKit/536.26 (KHTML, like Gecko) Version/6.0 Mobile/10B329 Safari/8536.25
.net Request.Browser => Height : 480 / Width : 320
51degree.mobi => Height : 480 / Width : 320
Telerik => Height : 568 / Width : 320

Navigateur Galaxy S3 (émulateur)
Request.UserAgent : Mozilla/5.0 (Linux; U; Android 4.3; en-us; Samsung Galaxy S3 - 4.3 - API 18 - 720x1280 Build/JLS36G) AppleWebKit/534.30 (KHTML, like Gecko) Version/4.0 Mobile Safari/534.30
.net Request.Browser => Height : 1024 / Width : 600
51degree.mobi => Height : 1024 / Width : 600
Telerik => Height : 0 / Width : 0

I'm very suprise of the Telerik results, which are never equals to .net native Request.Browser or 51 degree.mobie Given Screen Size, and are often smaller
Is it a bug ?
Could you explain it ?
How does telerik framework calculate those sizes ?
Thanks