Changing object dimensions for different resolutions

Changing object dimensions for different resolutions

I am a complete newbie at objective-C and have somehow managed to hack my
way through making my first app. Well almost.
I built my app 4inch/retina and am now making it backward compatible to
3.5. I spent a couple hours messing with constraints and couldn't seem to
get it working. My question - is it possible to change an object
parameters according to phone resolution with code? Something like...
if(3.5 inch screen){
object.height= 250px;
object.x= 250px;
object.y= 250px;
}else{
object.height= 350px;
object.x= 250px;
object.y= 250px;
}
Thanks for any help.