Hello G.,
Here is a first proposal, with partition (Yes it is a little obvious...):
@startuml
<style>
partition {
FontSize 20
}
</style>
skinparam activityFontSize 20
skinparam activityDiamondFontSize 20
skinparam activityArrowFontSize 20
group Some feature #1
:Variable #1 assign;
group Some sub-feature #1
:Variable #1 assign;
end group
group Some sub-feature #1
:Variable #1 assign;
end group
end group
@enduml

But you can change size directly for all the activity as:
@startuml
<style>
activityDiagram {
FontSize 20
}
</style>
group Some feature #1
:Variable #1 assign;
group Some sub-feature #1
:Variable #1 assign;
end group
group Some sub-feature #1
:Variable #1 assign;
end group
end group
@enduml

If that can help,
Regards.