How long were the priests’ breeches?

Answer:

"And you shall make for them linen trousers to cover their nakedness; they shall reach from the waist to the thighs" (Exodus 28:42).

This verse was discussed in the following articles:
What is Wrong with Mixed Swimming? 
Is it okay for boys and girls to swim together?
Clothing
Modest Apparel
Proper Attire: Growing Up in the Lord (Boys)
Proper Attire: Growing Up in the Lord (Girls)

Just how long were the priest's breeches? Did they reach from the waist (or hips) to the top of the thighs, making them equivalent to modern-day briefs? Or, did they go from the waist to the knees, covering the thighs and thus being equivalent to modern-day long shorts? Or, were they something in between, such as modern-day boxer shorts?

Why do we care? One point that is consistently made in the Scriptures is that clothing is to cover a person's nakedness. Nakedness is the exposure, directly or indirectly, of a person sexual organs. People often want to know how short is too short, especially when we are dealing with swimming outfits.

Exodus 28:42 is often cited because even though the priests were fully clothed in tunics (long shirts that reached to the knees or below), they were to wear miknac under their tunics. The article of clothing is described as reaching from the waist to the thighs. The word for "to", adh in Hebrew, can either include or exclude the endpoint. For example, in Genesis 11:31, traveling to Haran meant that Haran was included in the journey, serving as the endpoint. In Joshua 12:3, when describing the extent of a tribe's territory, it is described as going to the Sea of Chinneroth to the Mediterranean Sea. Here the endpoints are the limits and are not included. Hence, the definition of "to" does not solve the question of whether the thighs were included in the coverage. Therefore, we turn to the definition of miknac.

Josephus (AD 37 to 101), in Antiquities of the Jews, III:vii:1, said, "and, in the first place, he puts on that which is called Machanase, which means somewhat that is fast tied. It is a girdle, composed of fine twined linen, and is put about the privy parts, the feet being inserted into them, in the nature of breeches; but above half of it is cut off, and it ends at the thighs and is there tied off." In other words, Josephus is saying they were half-length pants that were tied both around the waist and at the low end of the thighs.

In the Complete Biblical Library, under entry 4507 mikhnas, in the Hebrew dictionary, it is stated, "Of the priestly garments, the underwear was primarily functional rather than symbolic. They extended from the waist to a little above the knees and were worn as an undergarment for reasons of propriety."

In the Illustrated Bible Dictionary, published by Tyndale, under Dress: Dress of priests it says, "The ordinary priests wore during the liturgical service a cloth which covered the hips and thighs."

An interesting passage to include in our consideration is Isaiah 47:2-3: "Take the millstones and grind meal. Remove your veil, Take off the skirt, Uncover the thigh, Pass through the rivers. Your nakedness shall be uncovered, Yes, your shame will be seen; I will take vengeance, And I will not arbitrate with a man." Uncovering the thigh is equated to exposing a person's nakedness. The thigh is the upper portion of the leg from the knee to the hip.

There is a Hebrew word for a garment that only covers the loins, the pubic region from the waist to the hips, but not including the thighs. The word is chagorah and is used in Genesis 3:7. Adam and Eve, even though wearing these garments, hid from God because they were naked (Genesis 3:10).

Just in case anyone wants to argue that covering the front side of the body is the primary concern, a passage from Isaiah tells us that exposure of the backside is equally shameful. "So shall the king of Assyria lead away the Egyptians as prisoners and the Ethiopians as captives, young and old, naked and barefoot, with their buttocks uncovered, to the shame of Egypt" (Isaiah 20:4).