OK, so looking at the Rat Guide, the dosage for Dexamethasone Sodium Phosphate is 0.23 mg/lb to 0.9 mg/lb. Sid weighs 755g which is 1.661 lbs. Therefore, his dosage range is 0.38203 mg to 1.4949 mg, which at 4 mg/ml works out to a range of:
0.1 ml to 0.37 ml, BID (twice a day)
You are also aware that you can't just stop Dex cold turkey, right? If it is more than one dose, then it should be gradually decreased over a specified time.
If you can talk to your vet tomorrow, it may be wise to wait if Sid's not in crisis.
For my Dixie, we did three days of Dex BID, then three days SID, then every other day for three more doses. That was at the beginning of treatment. She has not had any more Dex since.