If you can update with short entries, but not long ones, your computer's MTU (Maximum Transmission Unit, the maximum amount of data that can be sent in one packet over your network) may be set too high. This situation is most commonly found when your computer is set up to share an Internet connection with another computer in the house, or if you connect to your Internet provider over PPPoE (PPP over Ethernet, common on ADSL connections). Information on finding the proper MTU value for your Internet connection can be found by performing a query for "find maximum MTU setting" with your favorite search engine.
InsaneJournal can't help you resolve the problem; if you cannot resolve it yourself you will need to contact your Internet service provider or network administrator, or consult the documentation for your operating system.