Most single-speed bikes(particularly BMXes) use a chain that is 1/8" wide. Chains are always bought a bit too long, and then cut to size. Use a chain breaker tool.
No one can tell you how many links you need, it depends on the chainwheel/sprocket combo and the size of the frame.
Pretty much whatever you feel like. Do you want to go faster, but can't spin the cranks any faster? Get a smaller one. Do you feel like it's hard to get the bike moving? Get a bigger one.
Go for a SS (singlespeed) chain. It measures 1/8"
A nine tooth sprocket is a great combination
a 25 tooth sprocket will fit on a 9 tooth driver
You can get a 9 tooth sprocket, you will need a conversion kit to do it. Haro makes a decent one just do a search for Haro 25/9
9 tooth driver
25/9 gear ratio
a 9 will do but an 8 would be better
no you can not 25 with 8
25t means that there is 25 of the little spikey teeth on the outside of the sprocket :]
you never need a new rim its only the hub you change
It depends on the length of the chainstays on your frame.
20 inch upto 25 inch
calculating sprocket RPM is very easy if you know what the RPM of the drive sprocket is. If you know that then it is a simple fraction problem. If you make the drive gear the numerator or the top part of the fraction and the driven gear the denominator or the bottom part of the fraction (DRIVE/DRIVEN) I.E. if the drive sprocket has 25 teeth and your driven sprocket has 100 teeth (25/100) that will reduce to (1/4) or a 4:1 ratio. So if your drive sprocket is spinning at 1000 RPM then your driven sprocket will be spinning at 250 RPM. You can plug your specific sprocket tooth count into that equation and come up with the right answer.
8 indian size