! Welcome to the forums! I usually wait until someone with actual experience comments but it's been a while so I'ma go ahead and throw my 2 cents in. Generally speaking we recommend a program called armorsmith for scaling needs. It's about 35$ (I think) and it allows you to put in your actual body measurements and creat a virtual mannequin of yourself to which to scale the armor. You can then import your files and scale them directly to yourself. I use the program to scale pdo files for pep but I know some others who use it for 3d printing as well.
As for a tutorial, we do not have one for iron man armor. That being said, you can always look through other people's 3d printed builds and get a feel for their strategies and techniques. You can check the tutorials page(?) Section(?) For some guidance on 3d printing otherwise I would go to YouTube. Also check out the many 3d printer help threads if you run into questions. The best way to get help, however, is probably just to make a build thread for the armor and ask questions as you encounter problems. There are lots of members who are awesome at 3d printing and always willing to help.
For how long- couldn't say. Print time depends on your settings, material of choice and how effectively you have your printer dialed in. Total build time largely depends on how in depth you want to go, how many struggles you encounter and how much time you have per day to invest in this project. Some people can crank out suits in a couple of months, others it takes years. 3d printing is nice because so much of the time is passive but sanding and finishing is not a quick process - especially this time of year where it can be difficult to be outside.
Make sure that you choose your materials well and have whatever safety equipment you will need. You don't need some of those fumes in your lungs.
Iron man armor is dope so I hope you use this as a build thread and start posting pictures! Good luck, and good crafting!