Minimum length of 5 mm NMR tubes that can be used is 16.5cm.
For low field NMR (such as 300 MHz), you may use Norrell High Throughput 5 mm 7" tubes.
For high field NMR (such as 500 MHz), you may use Norrell Select Series 5 mm 7" tubes.
It is possible to get specialised NMR tubes such as:
The following may be of use (please contact your own agents for detailed information):
New Era manufacture NMR sample tubes & accessories.