As I read on the velvet manual
If the insert length of a library is unspecified, Velvet will attempt to measure it for us, based on the read-pairs which happen to map onto a common node.
So I leave the insert length blank but base on the same manual page it said
You should check the Velvet console output to make sure the insert length estimates are not absurd.
Where and How should I check the velvet insert length estimated!?
when I run velvetg without specifying insert length, the standard output captured in the nohup.out file contains the following lines. ......
[1564.566048] Computing read to node mapping array sizes
[1572.478116] Computing read to node mappings
[1593.435209] Estimating library insert lengths...
[1627.668440] Paired-end library 1 has length: 570, sample standard deviation: 43
[1634.073744] Paired-end library 2 has length: 244, sample standard deviation: 57
[1639.688635] Paired-end library 3 has length: 238, sample standard deviation: 54