Use a command block or chat command to target the dog
In Java Edition, dogs are wolves, so use `@e[type=minecraft:wolf]`
To make the wolf small, use:
`/data merge entity @e[type=minecraft:wolf,limit=1,sort=nearest] {Age:-2147483648}`
To make a wolf appear as a baby, use:
`/summon minecraft:wolf ~ ~ ~ {Age:-2147483648}`
To shrink it with a mod or datapack, use a custom scale attribute if available
In Bedrock Edition, there is no vanilla command to make a wolf smaller
Use a resource pack, addon, or mod for actual size changes