I need the settings for the various various wheel sizes for my BC 1200 bike computer, since in ghana it is difficult to get a particular wheel size as and when you need it.

I already referred you to Google and to Sigma support, but it sounds as if you probably need to replace the unit. It should work when you replace the battery, even if you have not set wheel size, as all bike computers come with a default setting. There is no way to "fix" such an inexpensive device (under $25 with shipping)
https://www.shop.com/Sigma+Topline+Wired+Bike+Computer+BC506+05160-1484064483-p+.xhtml?sourceid=298&CAWELAID=120179650032541809&CAGPSPN=pla&CAAGID=18133145218&CATCI=pla-152669075578&catargetid=120179650019543638&cadevice=c&gclid=Cj0KCQjw24nNBRChARIsALldLD0RZb-nYYKOoe52_lIzut84VQ406pw9AatsMu2TGjtjIT117rR1mx0aAoOwEALw_wcB

I’m happy to help further over the phone at https://www.6ya.com/expert/donald_f2ed37026a3ac881

Your "question" is too cryptic to understand clearly, but if you actually mean "I need the figure to input for wheel size for a 28 inch wheeled bike on an Orion bike computer," then try one of the many suggestions below:
bike computer wheel size Google Search

I’m happy to help further over the phone at https://www.6ya.com/expert/donald_f2ed37026a3ac881

Here is the user manual:
http://www.sigmasport.com/us_media/produkte/bikecomputer/topline_2009/topline_wired/bc1009/allgemein/visuals/manual_bc1009_us.pdf

If you care about accuracy find the manual, go to page 14/28 and do the roll-out (B) option.

http://www.sigmasport.com/en/service_center/produkt_support/bikecomputer/

the manual explains it...
http://www.sigmasport.com/en/service_center/produkt_support/bikecomputer/

If you want accuracy you should do a real roll-out measurement.

The manual explains how to measure your wheel exactly and set it into the computer...

http://www.sigmasport.com/en/service_center/produkt_support/bikecomputer/?flash=1&typ=bikecomputer

Page 28 and 30.

you can measure your wheel by placing your bike on a flat surface with the valve stem on the wheel in the bottom most position or 6 o clock position. mark the floor where the valve stem is. now roll the bike forward so that the wheels gets one full revolution and the valve stem comes back to the 6 o clock position and mark the floor at the valve stem. now get a tape measure and measure the distance between your two marks. Convert this measurement to usually centimeters. that will be the number that put into your computer.

Rick

Bicycle Parts and Accessories

you can measure your wheel by placing your bike on a flat surface with the valve stem on the wheel in the bottom most position or 6 o clock position. mark the floor where the valve stem is. now roll the bike forward so that the wheels gets one full revolution and the valve stem comes back to the 6 o clock position and mark the floor at the valve stem. now get a tape measure and measure the distance between your two marks. Convert this measurement to usually centimeters. that will be the number that put into your computer.

the manual can be downloaded here

Rick

Bicycle Parts and Accessories

Push bottom button until you see TRIP in bottom left corner of screen. Then go to the back, and push the small button and hold for about 3 seconds. The WS in the top will be blinking, and the number will be blinking. Push the bottom button to scroll through until KMH is blinking. Then push the top button, to toggle to MPH. Now... you will have to rest your wheel size on this screen (the numbers) go to Sigma sports website and get the manual, then look at your wheel size on the side of the tire, put in the correct number for your wheel size. Once all this is done, then turn over and push the small button for about a second for your changes to take effect. the screen will not be blinking now.

Dear spinnerrr

• Multiply the wheel diameter in mm or cm by 3.14.
• Measure the distance travelled in one complete turn of the wheel.

this is the four digit number you are looking, it should be between 2145 and 2149

Regards