Scope is effectively constrained by what you can see on the screen in a Trello board view. To maximize scope for a collaborative project, or to figure out when the board has reached a threshold that requires splitting it into two or more, the following approach may be helpful. [Note that boards can be copied and cards can (now) be moved to other boards.]
Since cards can be filtered by a max of 6 colored labels ('plus' one no color-no label default) and by members (names of people assigned to the card), when starting a board it's important to start labeling with these limitations in mind - before you put in a lot of effort.
In the absence of a status field for each card on a collaborative project board, the colored labels can indicate the status of what needs to be done; for instance no label (default) = to do, orange = doing, red = priority, purple = needs sprint, green = done etc. Then your cards can be filtered by what needs to be done and who needs to be doing it.
Choosing labels that can be the same on all your different boards makes them more efficient for team members to collaborate on; and if desired, for clients to keep updated on progress. Note too that when checklists are used in a card, a status bar will appear to indicate progress as items are checked off.
When creating a template board for projects, progression can be displayed by organizing the lists as project stages. Cards can organize tasks chronologically within each list, or by priority from top to bottom (per the Trello team's demo board).
On my laptop screen, 'maximum scope' potential then works out to be only about 8 lists and 6+ cards. These are effective chunks of information to perceive, track and manage from a cognitive science perspective (no more that 2x4 chunks) along each axis. This may seem small but one of the benefits of Trello is that it forces simpler, cleaner project collaboration and organization.
If much larger scope, greater detail, more fields, resource accounting, time management or sequential paths for tasks are required for your project, then you may have to ante up for more complex and powerful project management software or apps.
