using a sim module for sending more than 1kbytes throught htpps
HTTP is layered on top of TCP. TCP is a connection-oriented protocol &mdash; you establish a connection, you send data through that connection, and then you break the connection. Once the connection is established, you can send as many bytes of data as you like, using multiple AT+CSOSEND=0,0,"...." commands if necessary. The underlying TCP protocol will make sure that the stream comes out the far end in the same order that you put it in at your end.

You don't need to worry about the maximum packet size (MTU in IP-speak) at all. The lower level layers in the protocol stack hide that detail from the upper layers.