Depends on whether you are trying to keep the symbol approximately the same size (in which case Kev's suggestion could work) or simply scale up the symbol until the circle thickness is as desired (in which case Mike's suggestions could work).
If scaling is your preference, you could do a bit of math and simply set scaling amount accordingly. For example, if the circle thickness of a 24pt symbol is 0.02" and you want a larger symbol with 0.5" thick circle, then you want to scale up the symbol 0.5/0.02 or a factor of 25. So, either change the font size from 24 to 24*25=600 or multiply the symbol height and width by 25 (with the aspect ratio locked, click the object size width box so cursor is between the number and unit symbol ('"') and type '*25<enter>', and you will get a much larger symbol whose circle is now 0.5" thick.
You don't even have to actually do the math yourself, since you can replace '*25' with the calculation '*0.5/0.02' and CorelDraw will do it for you. Alternatively, you can click the font size and do the same thing there (click after the 24 and type '*.5/.02' to have CorelDraw calculate new font size of 600).