In a messaging system for 1000 users, the factors that will significantly affect the message storage space requirements are the maximum number of messages allotted per user and the message format used. The number of messages allotted per user determines how much space will be allocated for each user's messages. The message format used, such as audio encoding formats, can vary in size significantly - for example, a compressed format like GSM will use less space compared to an uncompressed format like G.711.