DeeP has the most complete statistics dialog box around. Besides the common total number of each object type, your level has an exact count by type and difficulty of all interactive Things. This is useful to make sure you made a balanced level. If you are just starting out, review the ratios in levels you thought were well done to give you some background for make these important decisionsJ
For larger levels, the estimated Save Game File size is shown.
(Note: the newer ports of DOOM no longer have a limit.)
If the total exceeds 180,000, you may have trouble saving. The number is approximate since events that occur while playing will change the size required. For example, a barrel that explodes no longer counts, but enemies that drop things when they die add to the total as well as the flaming heads generated by the enemies.
The easiest way is to reduce the number of Things. The next best thing is to pack Sectors (see File menu, packing). Reducing the number of things is normally suggested.