CA Datacom does not manage the placement or extent sizing of the LXX when it is initially allocated. In order to have a single extent in the LXX of 500,000 tracks, it is necessary to pre-allocate this as desired, preferably using the DSNTYPE=LARGE option.
Once the file is allocated, run DBUTLTY as usual to INIT the new LXX (assuming the previous LXX has been spilled to the RXX and the MUF has been shut down). Here is a sample input for DBUTLTY to do this:
//SYSIN DD *