A number of my fellow allotmenteers and I all manage to grow good sized fruit and veg with solely organic practices, but size isn't, to be honest, my first, or even fifth, priority.
Nurture your soil, give it the tools to feed your plants better than any other way can work, and you'll have the flavour and nutrition that'll compensate for any size issues.
Remember, in many cases SUPERMARKET VEG CONTAIN HALF THE NUTRITIONAL VALUE OF G-Y-O, so there's STILL more goodness in your veg, even if they are smaller.
Because there's only my wife and I in our household, I tend to grow smaller varieties, but I bet that if you add loads of organic matter to your soil, brew compost tea and encourage a large worm population in your soil, with leaves, paper etc, the size issue will take care of itself.