Both of my dog's weights hover around 20 lbs, sometimes a bit over. Should I choose the small or medium size product?

ASIN	B08P3CYX3H
ASIN	B08P3CYX3H
Age Range Description	All Life Stages
Best Sellers Rank	#38,467 in Pet Supplies ( See Top 100 in Pet Supplies ) #99 in Dog Flea Drops
Brand Name	Frontline
Breed Recommendation	Medium
Color	Blue Box
Customer Reviews	4.2 4.2 out of 5 stars (437)
Date First Available	November 23, 2020
Date First Available	November 23, 2020
Directions	1. To REMOVE Applicator: Prior to open separate one blister by tearing along perforation. 2. TO OPEN, use scissors to cut along dotted line of single blister, then pull apart foil and remove the applicator. OR fold on line at corner, tear and open at slit and remove the applicator. 3. HOLD Applicator Upright away from you and your dog’s face, cut applicator tip with scissors to open. Do not get pr…
Included Components	Treatment
Item Dimensions LxWxH	0.74 x 5.8 x 4.39 inches
Item Package Dimensions L x W x H	5.83 x 4.37 x 0.87 inches
Item Weight	1.13 ounces
Item model number	849
Item model number	849
Manufacturer	Boehringer Ingelheim Animal Health
Manufacturer	Boehringer Ingelheim Animal Health
Manufacturer recommended age	1 month and up
Model Name	Frontline Shield
Number of Items	3
Product Dimensions	0.74 x 5.8 x 4.39 inches; 1.13 ounces
Size	3 Count
Specific Uses for Product	Parasites
Target Audience Keyword	dogs
Warranty Description	Contact the manufacturer.

Shield Flea & Tick Treatment for Medium Dogs 21-40 lbs. Count of 3