The average tri-tip is usually about 2 1/2lbs. It will shrink some in the cooking process but trimming is typically not required. Look for a piece that has even marbling throughout the meat and no large pockets of fat. There will be a thin layer of fat on the lower side but not thick enough to trim off. I like to use the rule of thumb - 1/2lb of meat per person so a typical tri-tip after it shrinks a bit will feed 4 people. You can stretch this if there are girls or US Coast Guardsmen (sic?) present but don't get crazy.
Recipe is per average size tri-tip cut:
Mustard (You can use plain yellow but Dijon kicks it up a bit) - 6oz
Black Pepper - 1 tblspn
Onion Powder - 1/2 tblspn
Cayenne Pepper - 1/4 tblspn
Season Salt - 2 tblspn
Cumin - 1 tblspn
Garlic Powder - 2 tblspn
Chili Powder - 2 tblspn
Here's a trick - cut slices into the meat (not the fat side) about 1/4" deep and make about 1" squares. This really lets the spices get sucked into the meat. This is a reasonable option if you don't want to inject the meat. In this case, since we are not injecting with clear or near clear liquids, this is the best option.
Coat the meat with the mustard first - work into all the cracks. The mustard will help the dry rub stick to the meat - particularly when we rest (wrap in foil) the meat.
Work the dry rub into the meat getting as much even coverage as possible. If you made the slices in the top, make sure you get into all the cuts!
Cook the tri-tip at 250F grate temp - I start fat side down to get it soft and then flip it fat side up for the last 1/2 hour. Pull the meat at 140F internal but you can go to 150F internal for a medium cook. I wouldn't recommend exceeding 150F. Normally it should take about 90 minutes to get to 140 so flip the meat fat side up after about an hour. This will also give you the chance to get a probe temp of the meat and see how it's doing.
After the meat has hit the internal temp you are looking for, wrap it in foil and then a beach towel and place on the counter - or better yet - in a dry cooler. You can add a splash of apple juice at this point but if you have a nice piece of meat, you shouldn't have to.
Let the meat rest for at least 1/2 hour. If you have it in a cooler it will maintain temp for a few hours.
Unwrap and slice - try to get your slices about 1/4" thick.