Congo tetras - max size

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I've got 9 3-3.5" long congos in a 38 gallon tank at the moment. I'd like to eventually move them into a large tank and am wondering how big they can get. I've heard different reports. I've heard anything from 3" to 6" for max size. I somehow dont buy the 6".

Anyone had congos in a large tank for a long time?If so, how big did they get? Can large congos do fine with large silver dollars (say 5-6" long ones)?

Thanks in advance :)