Females are smaller, or at least they should be.
I've always had unneutered males, my breeder explained that they are big babies and at the time I didn't understand this, but now some 25 years later, it's been true. I like Big Babies. Some will say they are easier to train (generally), recall that girls in the animal kingdom have far more responsibility than the boys, so they are just more serious and "tougher" generally, although with all those characteristics that make them so uniquely Vizsla. And there's something about that which either works for you or it doesn't.
I fell in love with my first one, a boy, and just repeated it..although at first I would have taken either gender....but frankly it's the temperament of the individual that matters most, that varies widely form pup to pup and is easily discernible by about 6 weeks. So, be sure to tell your breeder as clearly as possible what you intend to do with the dog and what your specific temperament requirements are, and let him/her choose for you. Regardless of the gender, you want a pup that matches your requirements.
As an FWIW, recall that females go into heat every 9 months or so, and although I am vehemently opposed to desexing either gender...for health reasons....at the very least, if you cannot tolerate a 3 week heat for at least a couple years (The minimal time to wait for spaying), then a male might be a better choice.