h, I was walking downtown, trying to sort out a new passport, because other one has fallen apart, (nz passport), and I notice two female backpackers walking towards me, one was complaining to the other that her back was killing her, no wonder she had a 80 = 20 ltr pack on, the other one just replied, "I told you it would"
so save yourself the pain, and go light, or else someone else will be sayiny TOLD YOU SO
Your not mad in the slightest. 35 is plenty, especially in a warm climate. Just make sure it's a well built one (day packs aren't normally designed to be full/heavy/carried all the time), and also try and pick one which has pockets of the right size/shapes for what you specifically want to put in them. The other thing is to make sure you have decent loops etc on the side/top of the back to you can attach a jumper/ etc if required, or you pick things up on the way.
But yes, 35is more than enough.