I would strongly suggest that you read the script file that creates the data. For example, in this file:
fdtd_unit_cell_export_phase_field.lsf
the data file is:
EH_and_phase_vs_radius_interp_fdtd.mat
use the following script:
matlabload("EH_and_phase_vs_radius_interp_fdtd");
EE=E.E;
?size(EE);
result:
3 3 1 1 361 3
they represent x,y,z,f/lambda,phase and Ex/Ey/Ez.
Please check the data and see it is from which script file created and you will know the meaning of the dimensions.