After you plant flower or vegetable seeds, how often should you water them until they germinate and sprout? Does it matter much what size of seed it is, such as large squash or tiny herb seeds?

Depends in what you planted them.

If you planted them in a pot then you may need to put that pot in a drain pan and keep water in the pan until the seed sprouts - one it sprouts you water every other day from the pain letting moisture wick up. Everyday if you are in an arid climate, every other day if you live in an area of high humidity.

Peat pots need to be soaked prior to putting soil in them, then you keep the soil and pot damp.

In the ground it depends on your soil type. If you have nearly all sand you need to water every day. If you have high clay you need to water sparingly and just enough to keep the soil damp but without puddles.

If you made the perfect soil, a combination of clay, sand and composted material then watering every other day should do the trick, watering further apart as the plant takes root 'forcing' the roots to grow deeper.

Most seed packets will tell you how to plant them. Squash and melons prefer 'dry feet' thus they are planted in mounds. Corn prefers damp feet but are easier to irrigate via furrows thus you make a furrow, water down between the rows forcing the corn roots to grow down, the mound of the furrow is there for added support and to keep the base of the stalk from soaking in water (that can kill the corn).

Tomatoes should be started in doors in peat pots, when 6 inches to 9 inches tall you cut off the bottom 1/3 of leaves branch lets and bury the pot and the 'trunk'. This way you set the roots deeper and the tomato can endure longer periods without irrigation (watering)

Lentils and beans (green, peas, soy, lima, etc) should be soaked over night in a bowl of water, then kept moist.

Carrot and similar tiny seeds should ideally be covered with wet grass clippings or similar mulching material. I mow the lawn prior to planting carrot seed, Tossing the grass in a 5 gallon bucket and filling with water, each row of seed I lay on top of the soil and lay over that approximately 1/2 inch wet grass clippings. I water every day for the first week or until the carrot sprouts leaves, I cut back to every other day for another week then cut it back to further to 'water as needed' by pushing my thumb into the soil to the first knuckle - if the soil is dry at that depth I water.

All of these are factors that play into the answer.

keep moist till they sprout then dont let the soil get dry to much.