No probs with Angus dexter crossing. we use the angus all the time, 38 calves this year with no problems due to crossing. heifers and all.
the obvious thing to say is you must use an easy calving bull, but i would think an AI firm will only offer that anyway.
as for cow size, just not a cow that has not grown well herself, so nothing that has not thrived due to being ill etc. if you have a normal healthy dexter it will be no problem.
used limo, charly, beltie. trying wagyu and hereford this year to.
you get a tonnes better calf that grows quickly and is really commercial. the heifers make great cows too.
dominic