Rez the meditation space rezzer on the ground and touch it. A menu will drop down allowing you to choose 12 altitudes.
After you choose altitude, right click it and choose 'Sit Here'. It'll move up and rez the meditation particle space at the altitude which you choose.
You can also use the voice command to set the altilude on channel 66, as follows:
/66 VALUE (default value is 300) The command will look like this: '/66 300'