TECO servo motor 400W, with what size reducer?

That depends on how your gearbox selection criteria. Appearance requirements, precision requirements, cost-effective how to consider. If you want high performance you can choose the European production, but the disadvantage is that the price is expensive, long lead time. Next, choose Taiwan's, Taiwan's performance-price ratio is very OK, next if the precision requirements are not very large if you choose domestic, domestic prices are indeed good, the performance of the overall is not a high demand is no problem, but also convenient for you to replace later and so on. Next you will be the amount of your motor output torque calculated out, speed ratio calculated out. Participate in the selection of each manufacturer's reference book to choose on it.