@kashif18oc
you should be doing at Powerapp settings
please click on setting than display there is an option called Scaletofit turn it off. it will display the app in full screen

regarding blue bar its called as navigation bar unfortunately there is no way to hide the navigation bar by default in powerapp itself
but the work around is appending a queryparameter hidenavbar=true
Powerappurl?hidenavbar=true
queryparamters starts after ? mark if you have already someparameters than You need to use Powerappurl?somequeryparamters&hidenavbar=true
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