FSAirlines

I have always had an issue that it shows the wrong tank size, no matter what plane it will show more tank size on the left than the right. This therefor loads up the plane wrong and unbalanced.

In Latest version of Xplane and also the FS client

use "read only fuel method" and load the fuel in XP, not in FSairlines. NOTE TO USERS: THIS ALSO WORKS IN ALL SIMS WHEN A COMPLEX PAYWARE IS BEING USED THAT REQUIRES THE FUELING TO BE DONE IN THE PAYWARE PROGRAM VS FSAIRLINES CLIENT.

Go to OPTIONS then select READ ONLY - Then just fuel the airplane in XP, NOT in the FSA client and the client will read the fuel in each tank. The "max capacity" number becomes irrelevant.

Good answer Sam.
One thing to note with XP is we use the XPUIPC interface that is compatible with FSUIPC, in the MS Sims there is a identifying number for each tank that indicates its location and the order is

centre tank
left main tank
left aux tank
left tip tank
right main tank
right aux tank
right tip tank
center 2
center 3
etc...

But XP Plane does not follow that pattern, they could come in any order and I don't have a way to determine the actual position of a given tank, so the fuel tank positions in the client seam broken in X-Plane but its the best we can do. As Sam said, best advice is set the client fuel option to read only and set it in X-Plane before starting the flight in the client.