Aillio says minimum batch is 350 grams to have the right BT measurement on the Bullet R1.
With smaller batches the BT probe will not be sufficiently covered. But you can still roast smaller batches. Just ignore the BT reading.
Klaus investigated this with 100 grams batches to do sample roast. Read about his settings here.
Diego Cano in Colombia use the Bullet roaster to do a lot of sample roasting. He found that it works best in 250 grams samples.