The size would depend on the age. A fairly "old" looking carrot seed moth caterpillar on Bugguide was around half an inch. Newly hatched would be much smaller. How long are they currently? The markings and colours can change on caterpillars at different stages too which can make it difficult to identify if you can't find a picture taken at that exact stage (instar). This is also assuming they are caterpillars and not sawflies although so far I haven't found a sawfly that comes close. The pictures don't show enough detail to differentiate, we'd need a proleg count or a pic of one showing all the legs.
