in the on open event set the form's insideheight to the the height of the detail multiplied by your record count plus any header or footer height.
the statement will look more or less like this
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dim reccnt as integer
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dim dethi as integer
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dethi =320
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rccnt = ???
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me.form.insideheight = dethi*reccnt + me.formheader.height + me. formfooter.height
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you'll need to flesh it out by setting the record count variable, you can use an if statement to limit the size.
This should get you started, see where it takes you and post back