You can set and see the size of your tasks in relevant dartboards. Here's how.

To see the size of your tasks in your dartboards, choose the layout dropdown menu from the top right of your screen and then click the "+" to add the size property:

Once you are showing the size property, in list and roadmap layouts, you will be able to change the size by hovering over the size column in the desired task you want to adjust:

You will also be able to see your task's size below the task title whenever you open a task:

You can also choose to show size by default in any new dartboard you create. To do so, go to:

Settings > Properties > Size

From here, you will be able to toggle this property to show so it is always visible by default. You can also choose from different size formats, including T-shirt, Fibonacci, Exponential, Linear, or Free:

By default, sizes add up inside of subtasks. For example, if you have two subtasks of a main task, one with a size of "1" and another with a size of "2" then the parent will show size "3". You can also add a size to the parent task directly as well, which will increase the total.

For the purposes of summation, T-shirt sizes are considered akin to the corresponding Fibonacci sizes. (For example, XL corresponds to a size of 8.)

You can turn off this size summation rule in the property settings menu mentioned above by clicking the toggle next to "Sum from subtasks".