How to check if I am hitting the Express Edition size limit?
Here is a good script I shamelessly ripped from here:

use [Insert DB Name]

select
a.FILEID,
[FILE_SIZE_MB] = 
convert(decimal(12,2),round(a.size/128.000,2)),
[SPACE_USED_MB] =
convert(decimal(12,2),round(fileproperty(a.name, 'SpaceUsed')/128.000,2)),
[FREE_SPACE_MB] =
convert(decimal(12,2),round((a.size-fileproperty(a.name, 'SpaceUsed'))/128.000,2)) ,
NAME = left(a.NAME,15),
FILENAME = left(a.FILENAME,30)
from
dbo.sysfiles a


This is good because it will give you the free space in each DB file (you might have multiple files and perhaps someone set it up to put some objects in each) as well as the free size.

For example, you have have a 4GB data file which has 3GB of free space.  Maybe you have 1 MDF without a lot of data but a NDF with lots of data.  This query will tell you the free size in each file and what DB that file is allocated to.  Remember to add up all the 'SPACE_USED_MB' for each DB to get the total size.

Good luck!

Edit:
Removed a unsupported and buggy command that I thought I could get away with posting in here for this query. :(