The solution is to use Fortran 90/95/2003/2008, which has the capabilities needed for your problem, while FORTRAN 77 doesn't. Read the file once to determine the number of data items. Rewind the file. Allocate the array of the required length. Read the file again, reading into the arrays.
Using Fortran 2003/2008 (not tested):
use iso_fortran_env
real :: xtmp, ytmp
real, dimension (:), allocatable :: x, y
integer :: i, n
integer :: Read_Code
open (unit=75, file=...)
n = 0
LengthLoop: do
read ( 75, *, iostat=Read_Code) xtmp, ytmp
if ( Read_Code /= 0 ) then
if ( Read_Code == iostat_end ) then
exit LengthLoop
else
write ( *, '( / "read error: ", I0 )' ) Read_Code
stop
end if
end if
n = n + 1
end do LengthLoop
allocate (x(n))
allocate (y(n))
rewind (75)
do i=1, n
read (75, *) x(i), y(i)
end do
close (75)