Zac,
Another option, if you have the capabilities, is to take digital pictures of the fly at different steps and set up a powerpoint that could be seen by everyone. Have someone else step the computer through the steps of the powerpoint while you are doing the tying in front of the group, which will be fairly hard to see no matter what size you would be doing. Talk through the steps of the tying as you do them, and your audience will be able to see the results on the screen.
A wooly bugger or a wooly worm would both be fairly large flies to use, plus fairly simple and fast to tie in front of the class. Another choice might be a pearl marauder. Have all of your materials precut and laid out on a cardboard next to you for speed and efficiency.
And as your teacher has probably already told you, practice, practice, practice.
Good luck.
CJ (another teacher)

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The only limitations we have are the ones we put on ourselves.