Make sure you're using the newest gnuplot version.
If you read "help size" carefully you will see that size of the plot is
in relative units to the size of the canvas.
This apparently was different in older gnuplot versions and/or terminals.
set terminal pdfcairo size 17in,11in
set output "test.pdf"
# calculate the ratio of 8in to 17in and 10in to 11in
# need to add . to force floating point division
set size 8/17.,10/11.