A miniature is something smaller than in real life. One particular group of miniatures, Scale miniatures are miniatures with a particular defined size ratio in relation to full sized items. (HO trains are 1:87, dollhouses are available in a range of common dollhouse scales with the most widely available regular dolls house scale being 1:12 scale). Miniatures give us joy because they allow us to collect entire worlds in a comfortable area.
By the regular definition of a miniature being smaller than a normal sized object, and of no particular scale ratio, any elephant that fits in a regular house is a miniature, and a watch becomes a miniature clock.
"Miniature Paintings" have no fixed size below which they are referred to as miniatures. Portraits on ivory are most often referred to as miniatures, and the Hilliard Society defines the portrait head size for a miniature as below 2 inches. Miniature paintings are usually considered something below 12 inches in size, but particular painting techniques are also associated with the use of the term 'miniature painting'
"Miniatures" is also a term which has recently become associated with single drink servings of alcohol for mixed drinks, most often the single serving bottles sold on airlines. There are miniatures of vodka, but no miniatures of beer.
No set size but "to scale". Unfortunately miniatures aren't governed by any size rule, unless it is a scale size rule. We tend to regard things which have always been a particular size as full-sized and anything smaller than we are used to as a miniature. In this way, regular Royal Doulton figurines of people are considered normal size, (even though they themselves are miniatures) but Royal Doulton itself calls its smaller versions of figurines, miniatures. Many types of pottery, porcelain and glass do the same. There are miniature Caithness paperweights, miniature decanters, and miniature vases.
Is a toy a miniature? The same seems to apply to toys. If they are child sized, we consider them child sized, not miniatures. Most normal dolls aren't considered miniature unless they are a particular scale (usually smaller than playscale). There are full size toys which are miniatures, and miniature toys, which are smaller than regular toys. Many stuffed toy ranges are produced in miniature sizes.
If it is "to scale" is it a miniature? That depends on what it is and what scale it is built to. A miniature battleship or airplane could still take up a space the size of your house. Some miniature trains can only exist outdoors because of their size. We still call them miniature trains.
Miniature plants and animals are defined by particular sizes. There are currently more than 26 breeds of miniature cattle which must be less than 42 inches in height at 3 years of age. Micro mini roses must be no more than 6 inches with flowers, buds leaves, stems and thorns in perfect proportion to each other.
Rule of thumb says if it is smaller than normal, its miniature. Although the old twenty questions rule of is it smaller than a breadbox? probably still applies.