Flowerin

Should i start flowering my plant its a sativa and its 28 inches tall bt dnt got much space

28in sounds good.

I generally wait till the main stem is as big as my ring finger (big hands).

Since space is an issue, you might want to consider LST or (better in my opinion) supercropping.

Have fun.

Yes you can start to flower it. Mine are 28 days old today and im giving them bloom nutes for the first time today. Expecally if you dont have the room and you feel like its big enough, go for it. Normally you start to flower it when the pistols start forming a bud