fork size questions

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ok my giant fork bottom of the head tube to axel is around 19 inches my cousins mongoss is only like 18 inches. my brothers jr t bomber on his giant is 0ver 20 inches and steers like a chopper (it works, better attack angle it kills bumps) now i can get you guys exact numbers if you want but here is the deal. we have 2 bikes both getting forks my giant and a bm 29er. i assume smaller bikes get smaller forks right? maybe not but i dont want to drastically change the geometry on my giant yukon 21" if anything a tad longer would be ok. same for the 29er. so what is the deal i dont see specs for sizes when shopping for forks. so im wondering of there is such a thing? any help

If you want to keep the geometry of the bike as the manufacturer intended, buy a new fork with the same amount of travel as the original fork. For example, comparing your measurements above, I would say the Mongoose fork has the least amount of travel, your Giant has 1" more travel, and your brother's Giant has 2" more travel.

At one end of the spectrum, XC bikes are designed for leaning over the handlebar, and have a steep head tube angle and shorter travel fork.

At the other end of the spectrum, downhill bikes are designed for leaning back, and have a slack head tube angle and longer travel fork.

Putting a downhill (long travel) fork on an XC bike will likely cause it to have bad handling characteristics, and vice versa.

To answer your exact question (I think): For any given bike model, a large sized bike will have the same fork travel as a small sized bike.

If you want to keep the geometry of the bike as the manufacturer intended, buy a new fork with the same amount of travel as the original fork. For example, comparing your measurements above, I would say the Mongoose fork has the least amount of travel, your Giant has 1" more travel, and your brother's Giant has 2" more travel.

At one end of the spectrum, XC bikes are designed for leaning over the handlebar, and have a steep head tube angle and shorter travel fork.

At the other end of the spectrum, downhill bikes are designed for leaning back, and have a slack head tube angle and longer travel fork.

Putting a downhill (long travel) fork on an XC bike will likely cause it to have bad handling characteristics, and vice versa.

To answer your exact question (I think): For any given bike model, a large sized bike will have the same fork travel as a small sized bike.

correct. if you look at the geometry of the different frame sizes, the smaller frames have slightly different geometry to accommodate the same fork and retain the same handling characteristics. The changes are only very small though. You can usually get away with going up or down 20mm on the current handling without too much of an adverse effect.

Putting a downhill (long travel) fork on an XC bike will likely cause it to have bad handling characteristics, and vice versa.

The amount of stress that will put on the headtube...its not advised.