A lot of coupons will say, "not valid on travel size" or something like that...or they will say that the coupon is only good on a certain size (i.e. 8oz and up). When you see coupons that show no size restictions, see if you can find it in the travel section of the store! Big box stores have large travel sections that include not just personal care products but also laundry items. Yesterday I went to Walmart and picked up 2 of the Purex Complete 3-n-1 laundry sheets in the travel section. Each package contains 2 sheets and they were FREE with my $1 off coupon! PLUS there was a coupon inside for $1 off a full size product.
I already took the sheets out of the package and put them in a box that I already have of these sheets. I don't use them regularly but these are PERFECT to use when traveling. Even when we go to my in-laws house, and I figure I'll do laundry while we are there, I'll stick a couple of these in a gallon size ziploc bag and put them in my suitcase. Then I'm not mooching off my in-law detergent supply (not that they'd care) and I'm using a product I know I like.
I noticed Target also has packages of Tide in their travel section so that's something to check into as well. I've picked up travel size toothpaste for free as well and there are deoderants you can get for free sometimes also! Since the coupon says "any size" of that product you aren't doing anything wrong!
So really inspect your coupons when you get them and think outside the box as to how you can use them.