Locks up when deleting sent messages

[pwg]

I am cleaning up my sent folder. This does not happen every time so I can't
figure out if I'm doing something to cause it. I select a group of emails
to delete and hold down shift+delete to permantely delete the items (by
passing
the deleted items folder). Sometimes when I do this the screen will start
flashing and Outlook will lock up. It only seems to happen when I use the
shift+delete, but not all the time. Sometimes it happens when I select a
large group to delete but has happened on only a few emails too. Sometimes
there may be large attachments, but not always the rule.

Is this an Outlook error- been happening for a while? I don't get an error
message. I have to manually force it to close.

I don't know what manually compacting means- so I don’t believe I have done
this. When it locks up I'm trying to permantely delete (shift+delete) from
my sent box. I
occasionally empty my deleted folder which has other deleted items in it.

I don't know how to determine my .pst file size. If it is too big could that
cause the problem? Thank you for your help.