The panda Speedmaster is 40mm and the lug width is 19mm, an odd size but you can use straps you have for your Speedy on them. I've worn 20mm straps on a 21mm watch and the cap is barely noticeable; 0.5mm on either side.
honestly I've never seen this model in person but the sites I've looked at list the case diameter at 40mm, with one site listing the bezel diameter at 38.5mm and the case diameter at 40mm. Regardless, it's going to wear smaller than your subC. the SubC will probably feel bigger because of the bezel compared to this I have 7.5in wrists and personally the smallest I'd go on speedmasters is the 42mm like the original speedy pro. I'm more partial to the 44.25mm Speedmasters
You can use 20mm bands but if using with other 20mm watches too then will be loose on them as will shrink a bit. If you would use a 20mm on the 19mm Speedmaster Racing Speedies would suggest you dedicate it to it because of that.