Transfer Structure mapping

Ho Gurus,

I have created an KF for Amount Paid as type Amount. Looks like it has defaulted to length 17. I believe this could be the reason I get warnings when activating transfer structure.

While I have Amount Paid coming from R/3 which has a length of 13. I do not know how I can change from 17 to 13 in BW.

Any help is appreciated. Thanks.