Cite the scanned version of the original dicionary like this:
Toller, T. Northcote, and Joseph Bosworth. "mægen-stán." An Anglo-Saxon Dictionary: Based on the Manuscript Collections of the Late Joseph Bosworth : Supplement. Oxford: Clarendon Press, 1921. 627.
, es; m. A mighty stone or rock :-- Him on innan felþ muntes mægenstán (cf. þǽr micel stán wealwiende of ðam heáhan munte on innan fealþ, Bt. 6; Fox 14, 28), Bt. Met. Fox 5, 31; Met. 5, 16. Ðis synd ðæra xx hída gemǽro . . andlang wægæs óþ ðonæ mægenstán, Cod. Dip. Kmbl. v. 112, 18.