So I'm in the middle of a vh45 s13 build and want to build it with a single turbo. My intention has always been a big single. What about a 1.15 a/r gt45? I'm not too knowledgeable on turbo sizing but, the bit of information I can comprehend suggests it could be a good fit. But the people in here seem to have far more expertise than me

I ran a Garrett T04z with 1.15 for several years untill the Garrett ball bearing packed up. (And spewed particles through the motor during a 1,000km race so caused a rebuild!)
I gave the T04z to Steve Murch (he's in NZ but search him as he is a turbo guru) and he used the T04z exhuast housing and fitted a Holsett HX40 to it. He said he had decreased the actual size but as it's now a hybrid turbo a little hard to offer any more advice than that.