exhaust tubing

I got my 2/1/2 in and 2/1/2 out mufflers on order,so I am just wondering what size pipe to use either 2 inch or 2/1/2 from my headers back is there a big difference in just 1/2 inch overall.And what do you all think about this,my plan is to buy some flexable pipe to bend from the header past the crossmember .get it bent the way I want then take it to a exhaust shop as a template than they would have the bend I want.At this time paying for toes back and forth is not in the budget,i still have to get the alinement,and safty done.So trying to do whatI can at home,nor can I afford prebent from summit and the like.So any tips would go a long way.O this is not a high HP car just a daily driver 318.