The '03-'09 transporter most commonly comes as a T28, T30 and T32, i think you do also get a T26, but for wfp it would be too small a payload. They will vary depending on exact model, but as a guide, the T28 has a payload of about 1000kg, T30 about 1200kg, and T32 just under 1400kg.
As someone who has direct experience of the disparity between a quoted payload and what you can actually carry, i would say a 500l tank for the T28, 650l for the T30, and 750l-800l for the T32.