Thank you for your replies!
After seeing what 25ml looks like, I pour a little less than that if it's something heavy and smoky like an Islay, and a little more if it's lighter like a Speyside (assuming an Islay can't be light and a Speyside can't be heavy... I don't have too much experience).
I have bought 6 different whiskies so far: Talisker 10, Laphroaig 10, Oban 14, Lagavulin 16, Glenlivet 12, and Cragganmore 12. I bought the Lagavulin because I really liked the Laphroaig and it's peppery smoky flavor, but I think I still like the Laphroaig better since the Lagavulin seems sweeter and not as peppery to me. Same with the Cragganmore, I bought it because I liked the Glenlivet but I actually like the Cragganmore a lot more in this case (I like the spicy/flowery/licorice flavors).