Hand-tearing into craggy irregular pieces makes for some nice texture variation, and leaves extra nooks for sauces/dressing to cling to.
If using them on a salad, they should be big enough should fit on a fork with other salad ingredients, but not be a bite unto itself. If using them in soup, they should be bite-size at the largest.
Fat?
The short version — Use high-quality olive oil.
Olive oil and butter are common options. Croutons made with olive oil will keep longer (weeks) than croutons made with butter (days).
This article argues strongly for using olive oil instead of butter, since it results in a more crispy exterior.
Spice mixes are an especially good option if you want to make a batch quickly. Good options — Italian seasoning, Za'atar, ...
Garlic powder or grated garlic.
If using a garlic clove or fresh herbs (e.g. rosemary or thyme), you could heat the fat and the fresh ingredients in a small pan. Afterwards, remove the garlic/herbs from the infused oil.
Crusty breads are good. Great options are ciabatta, baguette, and sourdough.
Also, this is a good way to use up heels.
Fun fact: The word 'crouton' actually derives from the French word for crust.
Any leftover good bread. Being stale is a good thing here. This is a great way to use up bread before it goes bad, whether extra home-made bread or good-quality store bought.
Bake at 350°F until golden brown, crunchy on the exterior but still a bit soft within. This takes approximately 10 to 15 minutes. They will continue to crisp as they cool, so you may not want to overdo it. Turn at least once.
Bake on an aluminum sheet pan; using glass/ceramic/stoneware leaves you with a less crispy result.
You can also cook them on the stovetop.
Freezing
They preserve well in the freezer. Just re-toast them as you pull them out.
Ratios
This is by weight, and percentages are relative to the amount of bread.