I need to repair some wood blinds.  its not a rounded crown, the width is about 3/16" or about 4mm or 5mm, length is about 9/16" or about 14mm.

and what does T18, T20, T50, etc mean?

T18, T25, T50 refer to staple size. "Arrow" brand sold at most hardware places will sell them. If you don't have the staple gun already, then you will need to purchase the correct one for the staple you want to  use. The 18 and 25 I use are rounded and used for fastening small sized wire and 50 is squared or flat and used in a hammer type stapler used in putting down roofing paper or sub floor parer on hardwood floors. Should be able to fine what you need on the Arrow web site.

Sounds like wood shutters, not blinds. I'm guessing you have a couple slats that have broke loose from the tilt rod. Go to the yellow pages and look for a company that sells and installs shutters, they should have staples to make the repairs you need. Then all it takes is some smooth talking to get your hand on them.

Can't answer your question about staple size though

The 'T' numbers are the spacing in hundredths of an inch inside the staple.
T25 ia 1/4 inch (25/100)
T50 is a half inch (50/100)
Each width is available in a number of actual lengths and flat or rounded tops in some sizes.
Rounded tops have been used for smaller cable fastening for a long time.
Phone wires, thermostat wires, etc.
I have all sorts of sizes accumulated over the years.

Arrow finally managed to heavy up some T50 staple strips so they work reasonable well in 'hammer' type staplers.
The last change appears to be a slightly better strip of glue on the top bonding the individual staples together.

yes, inside wood shutters, thanks all for the info on the T stuff I think I see what I need, a T18 that is 9/16" long

The T is width.
Different lengths are available in most size.
Commercial staples used for things like assembly go by measured width and length.
They also take specific guns, often air powered.