Hi Laura! Technically speaking a 17.5 in each hand would be the equivalent (we have 17.5 dumbbells in our gym but that is not a typical weight that I see most people purchase).
I would err to the lighter side, say a 12 or 15 pound dumbbell in each hand until you get comfortable with the move and master your form.
I find I can lift more weight and with greater ease when using a barbell for squats vs a dumbbell for squats. I'm sure it is because the weight is more favorably centered and balanced right in the middle of the body, vs the dumbbell which has greater liberty of movemement (making the move more difficult in terms of balance and leverage).
Good Luck!