People t-shirt size in their daily lives all the time without realizing it. I do it on the weekends. Every weekend I always make a mental list of what I want to accomplish. In the spring time my lists gets long. Sweep out garage. Organize garage. Put snowblower to bed. Sweep deck. Take patio furniture down from garage rafters. Put out the hoses. Put out the big flower pots. Plant flowers. Pick up the lawn. Fertilize. Clean the windows. Clean the screens. Uncover the air conditioner. Thats a lot things to do and some are easier to do than others or just take longer. The main thing is is I don’t do them all at once. I put the puzzle together in my brain and figure out a handful of things I can get done in a weekend. I don’t know how long it actually takes to do any of these chores, but I do know that sweeping out the garage is smaller than putting out the large planter pots (those suckers are heavy).
T-shirt sizing work for your agile team shouldn’t be any different. Try to compare the work to other work and give it a size. Then move along. Don’t fret. If the replacing of the soffit vents turns out to be harder than you thought (I had it pegged to putting out the hoses) no big deal. The soffit vents got replaced and live moved on.