When I teach IV skills to students and residents, I always start with this question: "What's the best size and site for an IV?"
I generally get myriad answers that involve considerations of c/c, VS, etc... and people generally want to go with 18s out of the gate.
My response to the question, though, is, "In an emergency, the best line is the line you can get... even with a 24 in the thumb, I can push ACLS drugs and RSI drugs and buy us time to get a more definitive line."
Don't become dogmatic in your approach to securing venous access.