I have some black khuli loaches and they grow to about 4-5inchs long although i don't know about the more standard striped khuli loaches, probably the same though.
There are 4-5 species sold as "khuli" loaches. Some grow larger then others. AS TP has suggested, if you assume they grow to that size, then you'll be okay stocking wise, but don't be too suprised if they stop short of that.