I'm no expert, but, given the age of this work, and that Spenser used a form of language ancient in his own time, I would say that the word would be syllablized as mea-sur-ed to fit the poetic meter. There is a bit more evidence by the spelling used in other stanzas: retird; conspird, (clearly 2 syllables by the spelling) And if all that fails, there is poetic license
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