It’s quite normal to use italics or small text to distinguish block quotes from main text, but I don't think the gain in setting quotes apart is worth the loss of readability.
If you use italics or small text, you’re in good company. In The Elements of Typographic Style, Robert Bringhurst names three ways to distinguish quotes from main text:
Typeface (usually italic),
Text size (usually smaller), and
But despite how normal italics and smaller text size are, there are plenty of typographic rules and psychology to go against the practice.
In Practical Typography, Matthew Butterick establishes that italics and bold should be used sparingly—never for significant blocks of text—because they're harder to read and wear down readers. He's not the only one; Steve Krug and Nielsen Norman Group have the same advice.
Bringhurst's explanation for using smaller typeface gets really complicated because smaller text usually uses smaller line height, which screws up your grid, so you have to do crazy things to compensate.
That's why I'm a big fan of indenting quotes to distinguish them. They're just more readable.
Unfortunately—and this is especially case if you’re putting your quote on a slide, or if your quote breaks across pages—an indent may not distinguish it enough. That's why I like a bit more ornamentation, either in the form of:
giant quote glyphs, or a
quote bar (see "modern quote").
For verse, you can further distinguish block quotes by following the rest of Bringhurst's advice,
Verse is usually set flush left and ragged right…. But to distinguish verse quotations from their surrounding prose, they should be (indented or) centered on the longest line (p. 41, parentheses mine).
It looks like this. It's good advice, but it may be a lot of manual work to implement with your tools (Photoshop, Powerpoint, MediaShout, Proclaim). Then again, if your slides tool can do it automatically, it would be awesome for lyrics slides (because flush left text is easier to read than centered text).
Okay, I'm gonna go live my life.