When choosing between a 36mm and a 39mm Seiko mod, wrist size and personal style are key factors.

• 
  36mm Case:
  
  Best suited for smaller wrists (under ~6.5 inches / 16.5 cm) or for those who prefer a more classic, vintage-inspired look. It's subtle, elegant, and wears comfortably without drawing too much attention.
  
• 
  39mm Case:
  
  A very popular and highly sought-after size—this is the most commonly used case size for Seiko mods. It works well on medium to larger wrists (approx. 6.5–7.5 inches / 16.5–19 cm) and offers a modern, sporty presence while still being very wearable for most people.
  

Personally, I will always go for the 39mm

It’s the size where everything just looks perfectly balanced—whether it's the dial, bezel, or overall proportions. In my opinion, it gives the cleanest and most refined look.