Here I compared below conditions,

The value of the _size field must be less than or equal to 5000. The value of the _size_to field must be greater than or equal to 1000.

When we filter like Min[1000] & Max[12000]

_size <= 1000
_size_to >= 5000

<?php
$filter_size_rang_min_val = 1000;
$filter_size_rang_max_val = 5000;

$meta_query = array(
    'relation' => 'AND',
    array(
        'key' => '_size',
        'value'    =>  $filter_size_rang_max_val,
        'type'     => 'NUMERIC',
        'compare' => '<='
    ),
    array(
        'key' => '_size_to',
        'value'    =>  $filter_size_rang_min_val,
        'type'     => 'NUMERIC',
        'compare'  => '>='
    ),
);

Please let me know if it works.