Just in addition to Andy's answer,here are some detailed steps on how to do it:
1.Locate %ExchangeInstallPath%FrontEnd\HttpProxy\Sync\web.config and %ExchangeInstallPath%ClientAccess\Sync\web.config
Find the maxRequestLength in both web.config files and change the value to a larger one which you'd like to apply to the ActiveSync limit.
By default it is 10240 KB(10 MB).
Find the MaxDocumentDataSize and also change its value.
By default it is 10240000 bytes(10 MB).
3.Restart IIS and see if the changes have taken effect.
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