Restarting a Publisher involved in Merge replication
Will transactions be safely cued on the Subscribers - I imagine this will potentially cause transaction log growth?

Transactions will be safely cued on the subscribers and transaction log growth will be negligible.  Merge Replication utilizes DML triggers along with change tracking tables in the publication and subscription databases that contains metadata to determine which changes need to be propagated.  So you might see some growth in the Merge metadata tables while the publisher is offline.

What is a reasonable time period for the Publisher to be offline - do I need to worry about expiry?

Subscriptions will expire if the they do not synchronize with the publisher within the publication retention period.  The default retention period is 14 days.  You can check what your current publication retention period is by executing sp_helpmergepublication at the publisher on the publication database and inspecting retention and retention_period_unit in the result set.  Another way to check your publication retention period is to right-click the publication in SSMS -> Properties.  On the General page there is a section labelled Subscription expiration.

If you restart the publisher and it comes back online within the publication retention period then Merge Replication will pick up where it left off.