you definitely did not give enough info, but my guess would be you're a good skier and since you're 5 10 you probably way at least 145, so my guess would be to get 178s based on my inferences
Really it depends on your preference - I am 6'1" 170 and have been riding 171 invaders for a few years and they are just fine. Next pair, however, I will probably bump up to the 178 for the extra stability and ease of tip and tail grabs.