Automatic Electric Guns (AEGs) have gearboxes inside which gears sit on bearings. In theory, the greater the diameter of the bearings, the smoother the gears should spin. Even more so, newer style bearings also incorporate ball bearings to make it even smoother.
The original gearboxes made by Tokyo Marui had 6mm bearings which work perfectly fine back in the day. As times have moved on, many guns are above and beyond Japanese legal power and are cycling much faster then they used too. As such, to meet this higher demand, larger and smoother bearings have been developed.
Many newer AEGs come with 7mm or even 8mm bearings built in but many still do not. Even then, bearings are not magical enchantments of +1 to smooth running; bearings are like any other engineering component, you have to work out what bearing suits your setup best.
Element makes ball-bearing type bearings in 7mm, 8mm and 9mm. 7mm is superior to stock, 8mm is said to be smoother still bit only really worth it in high speed / high power setups. There is debate as to whether or not 9mm is really that useful but that does not mean that is absolutely true in all cases.
Whether you need 7mm, 8mm or 9mm bearings is up to you. Element makes solid grade quality bearings in all three sizes for very affordable prices. When building a custom gun there are many choice internal parts but equally so there are plenty of ways to cut financial corners and Element is a fantastic source of cheap parts which do the job just fine.