It appears that the "maximum automatic open size" is hard-coded to a value of 500000 (bytes, I believe) with no way to override it. By limiting this, we nip in the bud any potential complaints of Java OutOfMemory upon trying to open a huge file.
To view the file from within SQL Developer despite this limitation, just use the File|Open menu. For those huge files, please use an external editor. And if you don't want to open files automatically in order to suppress the warning dialog, use Tools|Preferences|Database|Export/View DDL Options and un-check the "Open Sql File When Exported" box.
Are you certain the export file does not contain all the insert rows? That would be a bug unless you hit an OutOfMemory or disk full condition. I just tried your scenario on at 55000 row table that produced an export.sql of about 20MB. All rows were included.
If you don't mind me saying, hard coding it is a very poor idea.
You shouldn't code for bugs unless they're actually likely to happen. I've got a reasonable spec machine and never had any problems - there was no need to 'nip ii in the bud for me', thanks. Now I'll have to revert to a previous version.
Now I can't export even a modestly sized database because somebody decided that such-and-such was an appropriate limit.