As i planned to order smart flip cover for my ipad, Want to know my ipad screen size (by inch) ?

By convention, your screen size is measured between the diagonal corners of the visible/illuminated area of the screen - excluding any bezels or surround.

You can find your model number here:

Settings > General > About

Then you can lookup the details of your specific iPad here:

Identify your iPad model - Apple Support

You need to correctly identify your iPad model, using this support article:--> Identify your iPad model - Apple Support

When you find the correct model, tap on "tech specs for iPad xxxxxx...", and you'll get a complete list of its specifications.

Regards

Giulio

Magnetic smart “flip” covers from third party makers are normally identified for any Apple iOS device by year and generation/“A” model number of iPad and the actual diagonal screen measurement.

The 4 digit “A” model number can be found on the aluminium back of your iPad, in small print, right under the larger bold print “iPad”.

Such description as, Magnetic smart cover for iPad 2017, 5th gen & 2018, 6th gen iPad 9.7 inch screen models ( sometimes followed by the Apple 4 digit “A” model number ).