TravelonWomen's Anti-Theft Metro Convertible Small Crossbody Bag Cross Body, One Size

Q: How log is the strap?

I was able to wear it cross body

Q: Will the back pocket hold an Iphone 14 pro max?

Yes, but it won't be completely concealed in the back pocket. Eg. the length of the phone means it will fit horizontally but the width means that the top edge will show above the fabric of the pocket by about 0.5cm (according to the specs of the phone online). However, as it is the back pocket, it's against the body, there is no zip on that pocket to interfere with, and the hooks for the long strap are flat and will fold up if in use or down out of the way.