i made the mistake of washing my quilt top now i don't know how big my batting should be

Big as in height loft wise or quilt top wise? Quilt top wise I would iron the heck out of it. Some starch their's I have not then I believe it is at least 4 inches beyond the quilt top. Hope this helps. Jade

not sure i understand why washing your top has anything to do with the size of batting?
but guidelines are ...
batting and backing should be at least 4" (all the way around) larger than the top...so if your top is 60"x72" the batting backing would be at least 68" x 80".

If you have washed your top you really should wash or rinse your backing and batting. So everything has gone thru the shrinking process. Then, when you wash the finished quilt, it won't do odd things.

my quilt is 12x12 it is biger than my king size bed i've been thanking about taking some of the squares off

i was afraid the batting would fall apart if i put it in the wash by it's self

I never prewash my batting and had no problems. I agree that you should prewash the backing.

IF you decide to prewash the batting - I used to prewash the Warm and Natural when it still had the cotton nibs in it (years ago) and the key was to not over-agitate. I used to let it soak in a lightly soaped water - then rinse and spin - and dry in the dryer. No problems with that either.