I have a giant rincon from the 90's and I want to put suspension forks on them and keep the 26" tire what size forks should I be looking into getting
If the bicycle has a rigid fork now and this fork is not marked as having "suspension-corrected" dimensions, in particular, axle-to-crown length, then this bicycle is not officially compatible with any suspension forks. Simply put, installing such a fork will either raise or lower the front of the bike, which will affect its steering properties and ground clearance among other things. There may be other problems, like the new fork's crown touching the downtube when turned.
If the current fork is "suspension-corrected", then it should be possible to replace it with a suspension one. Bikes of the era rarely had suspension travel more than 100 mm. There are other compatibility parameters to consider though, like steering tube diameter and shape, wheel axle type, brake system etc.
A bigger issue is, however, that 26"-wheels and accompanying components are (unjustly IMHO) considered obsolete by the market. It is hard to get new good parts for them, as most of new MTBs are sold with either 27.5" or 29" wheels. Those few cheapest new bikes with 26"-wheels out there are coming with OEM forks (i.e. those cannot be bought separately), and those forks are not especially known for their stellar damping properties.
The dirt jump bikes are however fixed to the 26" wheel size. It should be possible to get an aftermarket fork with 80-100mm travel for them. However, as I mentioned before, they tend to have modern "standards" elsewhere, such as through axles, tapered steering tubes and disc brakes, things not known in 1990's.
You need to match the axle-crown height (on the loaded suspension), the Steerer (if early 1990's will likely be impossible to find quill stem, otherwise straight 1 1/8") Modern forks used tapered steerers so be carefull. You will also need to match the axle, which will be 9mm QR not through axle, and the fork wil have to have V Brake bosses unless you want to upgrade to disc brakes (means new hub which practically means a new wheel)
You can get new forks that will fit the requirements, but they are from lowest end range of what is manufactured today by low end manufacturers - cheap and generally pretty nasty. Quality forks from a donor bike upto early 2000's might be a better way to go (after that quality forks wen disc), but age will have taken its toll unless well maintained and not used much. Examples of used forks that fit the bill are getting expensive.
Ultimately, it fits into the fun(*) to do but not economical bracket unless you can find good used parts, which is the real challenge.
- Fun - something you do that is challenging, time consuming and costly just because you can.