The Spider is effectively a tall rest. Where a rest is required and the cue ball is just the other side of another ball, a cross rest will not do the job because the cue needs to go over the intervening ball. Such shots are only possible with a spider.
A spider will be required fairly regularly on a large table and occasionally on smaller tables - so ideally the minimum for any table is both a cross rest and a Spider.
Standard length spider for a full-size table is around 60 inches but this will be too long for smaller tables. The 48 inch shaft is more suitable for 6 and 7 feet tables and the 54 inch shaft works well for 8 and 9 feet tables.
We also can supply a long spider which is a non-traditional implement but can be useful in some situations or as a compromise solution for those who don't want to buy both a spider and a half butt.
Brass Spider Rest with 60 inch Ramin shaft
Brass Spider Rest with 48 inch Ash shaft
Brass Spider Rest with 54 inch Ash shaft
Brass Spider Rest with 60 inch Ash shaft
Brass Spider Rest Head (head only - no shaft)
ETA: 1 working day + transit time
Long Spider - 84 inch Ramin shaft with spider brass head