The ornamental design for the keychain-sized thermometer, as shown and described.
FIG. 1 shows a perspective view of the front side of the thermometer.
FIG. 2 shows a front view of the thermometer.
FIG. 3 shows a back view of the thermometer.
FIG. 4 shows a left side view of the thermometer.
FIG. 5 shows a right side view of the thermometer.
FIG. 6 shows a top view of the thermometer; and,
FIG. 7 shows a bottom view of the thermometer.
The broken line showing of a keychain ring is included for the purpose of illustrating NON-CLAIMED SUBJECT MATTER and forms no part of the claimed design.