It all depends on how you are going to wear it. I use three shirt sizes for layering.
light weight turtleneck...merino wool
XL Tall wool shirt....(Woolrich)
2XL TALL wool shirt...(KOM-lightweight)
3XL TALL Pullover...(KOM-heavy weight)
I wear my XL Asbell pullovers with the turtlenecks and/or the XL Tall wool shirt when I'm stumping, hiking, moving stands etc.
I always buy tall's even though they are to long because I hate it when I lift my arms up and the sleeves pull up...especially when it is cold.
I would find a shirt that fits you well and take the measurements...shoulder to shoulder, shoulder to cuff, neck to hem, pit to pit...and relay that data to Theresa.