You could set the params[:pagesize] to the count of all projects. Your view and controller might look something like this:
index.html.erb
<%=link_to "Show All", projects_path(:show_all => true) %>
projects_controller.rb
def index
params[:pagesize] = params[:show_all] ? Projects.count : 10
@projects = Kaminari
.paginate_array(@projects)
.page(params[:page])
.per(params[:pagesize])
end