Is one piece of bread sufficient for it to count as a “bread meal”?
One slice of bread is definitely enough to make your meal a bread meal.
If one ate a “k’zayis”- (lit. an olive worth) of bread it is considered a bread meal. An olive’s worth is about an ounce, and a slice of bread from the middle of the loaf is close to 4 k’zaysim. One should be careful to eat the k’zayis within 4-5 minutes, in order for it to be considered “eating” according to halacha.
Another point, we only say “al netilas yodayim” when washing for a bread meal if we will be eating a “k’beitza” the size of an egg- which is as large as two olives. Therefore, it is preferable to eat 2 kzaysim, in order to say al netilas yodayim.
Vzos Habracha pg. 371,