Planning for clothing sizes

FTM baby due next June. Am trying to take advantage of the holiday sales for baby clothes right now. We are planning to use GMD prefolds and Thirsties covers for our LO. How do I plan for clothing based on this? I’ve heard that I’ll need to size up on the clothes to account for the bulky diaps. How much do you recommend sizing up (1 or 2 sizes or more?) and will I only need to do this with pants/bottoms? What about onesies?

Thanks for your help!!