Hi Sue,
I've often seen it left in Italian (in italics) in art history texts. I suppose it depends on your audience. In any case, if you need to translate it (maybe in brackets) or qualify it, I'd use "medium-sized", as the paintings weren't really that small.
HTH
Sarah


Example:

"It represents the 'Sacrifice of Isaac' and is, with its medium size of 119 by 105 cm, a typical 'quadro da stanza', like the pictures now in Besançon and Greenville, which also depict biblical scenes."

From 'Some Unknown Works by Guglielmo Cortese' by Dieter Graf and Eric Schleier, published in The Burlington Magazine, Vol. 115, No. 849 (Dec., 1973), p. 797



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Note added at 21 hrs (2006-11-29 12:07:18 GMT)
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Here's another example:

The years from the later 1590s through the first decade of the new century to the 'St George and St Catherine led to Martyrdom' (no.78), from the 'Madonna della Ghiara' in Reggio Emilia (commissioned in 1618), in which Ludovico's art became increasingly spiritual and austere, were represented by a great number of altar-pieces, some medium-sized 'quadri da stanza', and a very few cabinet pictures..."

Review: Ludovico Carracci. Bologna and Fort Worth
by Erich Schleier
The Burlington Magazine, Vol. 136, No. 1093 (Apr., 1994), p. 263

http://links.jstor.org/sici?sici=0007-6287(199404)136:1093<2...

PS Be careful if you refer to religious/secular art, because while 'quadri da stanza' were obviously painted for domestic settings, the subjects were nonetheless often religious.