It's almost impossible to completely rid your sample of all ribosomal RNA, but you can clean your RNA until the vast majority of it is gone. The residual amount of rRNA left in your sample won't interfere with your reverse transcription. If your reverse transcription is good, you should get good labeling and a nice microarray. We take whole sample measurements down to 50 microliters and measuring concentrations as little as 1.6 micrograms. However, every spectrophotometer is different and accuracy is always in question.
Confidence is typically higher when measuring larger amounts.
We have also used Ribo-green to determine RNA yields. Yes, using the Trizol method. This was not a successfully isolation, however. A much better method is the Pine Tree method , used to isolate RNA from recalcitrant tissues. We need to see, as a quality control, that your mRNA is capable of being reverse-transcribed.