Manually set log file size after shrink SQL Server 2008 R2
Your file management can be a completely online operation.  You have two paths, depending on your need to retain your log information for recovery purposes:

Point in time recovery not needed


Convert the database to SIMPLE recovery.  Execute a checkpoint to write transactions to disk.
Flatten the log.
Resize the log to the appropriate size. 


I also recommend setting a fixed growth amount and unlimited growth (so as to help manage your log better).  Note, fixed growth amount is very much an it depends amount, I'd recommend going with 1-2 GB initially depending on how much growth that log could expect to see.  Ideally, your log won't grow much, so this shouldn't have much of an impact.  If your log is growing regularly, you might need to revisit your size.

Accomplished using:

ALTER DATABASE [foo] 
SET RECOVERY SIMPLE;

CHECKPOINT;

DBCC SHRINKFILE (foo_log,0);

ALTER DATABASE [foo]
MODIFY FILE (NAME=foo_log,SIZE=8000MB,MAXSIZE=UNLIMITED,FILEGROWTH=1000MB);

--Optional if you want the database in full recovery mode 
--for point in time recovery going forward
ALTER DATABASE [foo] 
SET RECOVERY FULL;


Point in time recovery needed

The biggest hangup will be that you can not shrink your log file past your currently active VLF segment.  To see this, you can use DBCC LOGINFO in the database context.  Any segment with a Status=2 is active.  To clear active segments, you will need to run a transaction log backup when no transactions are currently active in that segment.  Your steps are:


Run a transaction log backup.
Shrink your file. (Ideally flatten, but if your database is active this will be hard to do).
Repeat steps 1 and 2 until your log is an appropriate size, ideally as small as possible.
Resize the log to the appropriate size. 


Accomplished using:

BACKUP LOG [foo] TO DISK='&lt;Location of t-log backup&gt;';

DBCC SHRINKFILE (foo_log,0);

--Repeat the above until your log file is small "enough"

ALTER DATABASE [foo]
MODIFY FILE (NAME=foo_log,SIZE=8000MB,MAXSIZE=UNLIMITED,FILEGROWTH=1000MB);


Some additional resources to understand what's going on here:


SQL Server Recovery Models
Kimberly Tripp's t-log recommendations
Database Checkpoints