People generally use 500-1000 nl.
Here is an example from Peirs et al 2015
SNI was preformed as previously described. Spinal cord injections of AAV were performed at P9–P10 or P15–P16. Under isoflurane, midline incision was made without laminectomy and virus delivered (1 ul) slowly with a glass microelectrode (50 mm tip) between lumbar segments L4 and L5. Silk sutures were used to close lassimus dorsi, and skin and Ketofen was given before and 1 day after surgery. Behavior was tested 3 weeks later. For more details including virus titer, see Supplemental Experimental Procedures. @RPSeal
Similarly, this JoVe video from Inquimbert et al. uses 1 ul
If you can see the dye, it’s getting in. As far as titer goes, people are usually using 10^12 to 10^13 range. You will need to test yourself.
Here are some more resources: