Khubaib,
You can at some risk.
You can copy the FRX to a custom location and then go in and modify the font related values in the rows of the FRX (remember it's just a dbf with a different extension).
The risk is that the object sizes probably need to change if they change the font size very much. And if the sizes are changed the whole layout of the report might be disrupted.
The report designer is a re-distributable component. You can issue a MODIFY REPORT command from inside your app and the user can make modifications to it. You have to have somewhat competent end-users though to give them this sort of power.
If the report isn't too complex you can also render it out to HTML and then the user can easily use View/Textsize to give them some level of size changing. And of course you can gather a desired font/size and use that when rendering the html file.
>How i can add utility to appear a form before report. so user have a option to change the font name and size of font.