Here are the lockers sizes I found on the DIS:
Lockers
If you don’t want to carry those heavy backpacks and purchases, the
Disneyland Resort offers lockers in both parks and central plaza. Lockers located in the parks are smaller so if your plans include a picnic in the picnic area, you will need to store your items in the larger lockers located inside of the picnic area (outside food is not allowed in the parks).
Size/Price
Small 12"x x 18"h x 12.6"d - $7 All Day (All locations)
Large 12.5" x18 " x 19" - $10 All Day (All locations)
X-Large 15" x 25 " x 19 " - $11 All Day (All locations)
Jumbo 15" x 25" x 25" - $12 All Day (Inside picnic area only)
Super Jumbo 18" x 25" x 37" - $15 All Day (Inside picnic area only)
Locker Locations:
Disneyland Park
The lockers are located half-way down Main Street U.S.A. next to the Cone Shop.
California Adventure Park
Lockers are located just inside the park Main Entrance to your right.
Central Plaza/Downtown Disney/Picnic Area
Guests can use lockers outside the main entrance to both Disneyland Park and California Adventure Park to store larger items and items not allowed in the parks (food, large backpacks and coolers).
If you can't fit the bag on the ride, you will not be able to ride as CMs will not store packages.