He's on the big side...but I wouldn't say a giant. Do you know what he weighed at 6 months - generally that is 75% of what they'll weigh as an adult/finished growing (14 months). If his parents are from European or Autralian blood lines, that may explain it as they are generally larger than NA V's. If 26 inches at the withers than he does seem a little taller than most.
Mac is a week shy of 6 months and he's almost 20 kg's.
My 20month old male Vizsla is very very very tall. He is 27in at the withers and 26kg, which I think is quite light for his height, he looks very thin. He's all legs really! I've met Vizsla's who are just as tall.