awk

sprintf

:: in batch file
cat sizes.txt | awk "{foo = sprintf("""--v_x %%s --v_y %%s""", $1, $2); print foo}"

:: in cmd
cat sizes.txt | awk "{foo = sprintf("""--v_x %s --v_y %s""", $1, $2); print foo}"

size content :

38 38
27 27
22 22
20 20
17 18
17 16