I do a few icon theme works. For me, it is very important to switch to different icon sizes. ie, I am working on a 64 px icon and now I need the same icon in 32 px size. Someone suggested me, group first, enter the values on top and then use ctrl + shift + R. But I forgot where to enter the values and when to lock it. It will be great if someone can guide me through.

Select the icon with Selection tool . On the control bar, which is the first bar above the horizontal ruler, you can enter the W or H that you want for the selected object. Be sure to click the tiny lock icon which is between the W and H, to keep it in perspective. Then press Enter.

Thank you @brynn . I will try that.