What would the current state of human civilisation be, if humans were the size of the average lagomorph?

Here’s my conjecture.
.Brain size would be limited so processing capability would be limited too. Calorie intake needs would be reduced so a vegetarian diet would be easy to support. They would live in caves and naturally occurring burrows.
For fun lets assume they have the brain power of today’s human. They would need to protect themselves from predators like birds, coyote, snakes, etc otherwise they would be wiped out. They could either reproduce prolifically like mice and other rodents, or develop some form of weaponry fight off or kill the predators. The latter is the most likely scenario.
They might have spears with improved points. Maybe they would discover toxic plants and make their own poison darts. With active, capable brains they’d have all day to figure out how to protect and feed themselves. And opposable thumbs would give them the dexterity to put some of their ideas to use.

You’re fortunate that @LuckyGuy knows what you’re talking about. Since it’s you asking the question, my guess is that the average reader will assume you want a comparison between humans and one of your esoteric obscure fantasy creatures. If you’re looking for responses, why not ask about people the size of a rabbit?

The smaller size would mean a much larger percentage of body mass exposed to the elements. So there would be extreme variations in physical adaptations depending on climate. A higher metabolic rate would be required to maintain uniform body temperature meaning a higher caloric intake per ounce of animal. The animal would necessarily spend a great deal more time eating than you or I, and if vegetarian, more time still. In fact, the animal’s life would probably amount to little more than avoiding predation and finding enough to eat.

@stanleybmanly You bring up a good point about the ratio of body mass to surface area. They would either have a lot of body hair or wear some form of clothing to reduce heat loss. While their daily calorie intake would be lower than ours, in proportion to their size it would be much greater. Maybe they would eat more frequent meals or even eat/graze continuously
This sounds like I’m describing a Hobbit – albeit one that spends time making weapons rather than smoking pipes.

Size plays a role in gestational time, so @LuckyGuy‘s mention of rapid reproduction would be fairly easy without needing large litters. In that case, offspring would be expected to take on responsibilities rather young, and families would be a community within itself. While this allows for plenty of thought and effort available for meeting immediate needs, scientific advances would take much longer to develop.
I should think that population issues would develop two societies, the up, and the down people. Some would take to burrows, others to trees.

We would evolve into awesome sarcastic space aliens.
https://www.youtube.com/watch?v=vySgYadgKsw

Would there be more of them? Same total biomass and/or consumption rate, or less?

No civilization, just 24/7 fornication.

Would it be only humans that are that small, or would all other creatures and animals be proportionally smaller as well.

If, in your example, it is only humans that are small, humans would not exist after 10 years or so, because they would have been eaten by other, larger creatures.