[wbmkk]

This is my fist year growing chilli plants.

I sowed them in March, on the same day I sowed my tomato seeds, thinking they would  grow at the same rate (why I thought that, I don't know)

Anyway, they were in 3inch pots, until a couple of weeks ago, when I put them into 6" pots. I has no larger pots at the time, as all had been used for tomatoes.

Anyway, the chilli plants are now about 6" tall, so I was wondering when I should be potting them on, into their final pots. I have seen comments stating that pot size has a great effect on chilli crop, so I am keen to get it right. Should I pot them on now, or wait a few weeks.

[Eightball]

are there roots coming out the bottom? if not you can probably wait.

[New shoot]

I think that if the plants are only 6 inches tall, the pots they are in will do them for another couple of weeks at least. 

As Eightball says, look for roots coming out of the base of the pot and gently take one out of its pot to see if the roots have spread all through the compost.  If you feel the compost moving as you tip the plant, the answer is no, so stop there and put it back up the right way quick 

Chillies really don't like sitting in a lot of wet compost and if the roots are not established enough to fill the final pot pretty quickly, that is exactly what is going to happen