If you are going to do it at all, card sizing is too difficult to get right. That is, if we make it a complicated, objective, and refined process, it is hard to do consistently. So I would suggest a simplification of the sizing process. Just have three sizes. Lets call them Small, Medium, and Large.
Here is how we make it work, and make it consistent(ish).
- Put all the cards that need sizes together. This could be on the same Trello board or taped to a wall. Just so long as they are all visible together. If you have tons of cards (more than 20-30) you might need to batch your work so it will all fit. You want to be able to see all the cards at one time without moving (or scrolling).
- Create three columns. Small, Medium, and Large.
- Now, as a group, pick one card and choose a size. Don't worry if its right, just choose a size, and put the card in that column. When I say as a group, I don't mean a facilitated conversation, I mean everyone start moving cards together.
- Pick up another card and compare it to the card you just placed in a column. Then, place it in a column relative to the first card. If the second card is smaller than the first card and the first card is in the small column, you might need to move the first card to the medium column.
- Repeat the process of selecting a card and placing it in a column relative to the others until all cards are placed.
- Review the cards in those columns to ensure you didn't make a mistake.
- You are done.
I've used this process a number of times. It works very well in terms of getting like sized cards grouped together and it is also very fast. The process creates a consistency because it is a relative. We are not trying to find a size by using points or beans or whatever. And we are not trying to determine size in terms of time. We are just determining, relative to each other, what size is this card. The best part is, this process usually takes 10 minutes.
Throw away all the things you have heard about points or time in terms of sizing a card. Think about the sizes only in relative terms to each other. When you are done sizing, pick a numeric value for each size. I like 1, 3, 5, but you can use any set of numbers; the only rule being that they are numerically relative to Small, Medium, and Large.
Now, plug that numeric value into your tracking tool and use that to measure progress. You can get a velocity number from this, or a yesterdays weather report, and figure out how much work you can complete in an iteration. Or if Kanban like systems are your thing, you can use these numbers to determine how fast work flows in terms of size.
All of this comes together to make planning easier and less stressful. It also self adjusts to the teams development. If the team creates a template to speed up some work, the cards will naturally fall into a smaller size because when they determine the relative size they will think, 'Hey, we have a template, this is easy.' So there is no more going back to the history and checking ourselves.