Automatically Read Array Size into Idl

I am trying to automate a code in IDL that takes a few .dat files created and reads them into an array. Each of these files has 4 columns but a different number of rows, and I am not sure how to set it such that it automatically fits the array to the number of rows each time.

openr, lun, +field+'_'+color+'.dat', /get_lun
array = fltarr(4, 380)

readf, lun, array
outlier=array[0,*]
mag=array[1,*]
ra=array[2,*]
dec=array[3,*]
position2 = [outlier, mag, ra, dec]
free_lun, lun
return, position2

This is the current block of code I am using, and I have just been changing the array value per file, which has become tedious, and it won't work if the array value is set too high.

This next part is something I am curious about but it's managed to be worked around mostly okay, but I would still like to ask if it could be reworked similar to the other code:

I also have a similar code that created the original files im reading (sorry I know it's a little messy I am very new to IDL):

i=0
position = []
this=[]
OpenW, lun, +field+'_'+color+'.dat', /get_lun
while i lt 16000 do begin       ;typically 16798
outlier = plot_data[0,i]
mag=plot_data[1,i]
ra = plot_data[2,i]
dec=plot_data[3,i]
if outlier lt 0.0 then begin
PrintF, lun, outlier, mag, ra, dec

                                ;
endif

position=[outlier, mag, ra, dec]

i=i+1
endwhile

... which reads a large .sav file and converts it into a 4-column array so I can create the new file with the outliers I use for the previous code. I'm not sure how to open or read a .sav file so I was using an old code from the person who worked on the project before me, but the size of 16798 doesn't work for every single .sav file so I reduced it, but I do want to see if I can fix it so I'm not excluding ~700 points from all other data to accommodate a small number of other files. Is there a way to use a similar code that would fix the first code here?

Sorry, know it's a lot of questions and any help would be greatly appreciated!