I want to build my abs. I am eating well, and I wonder if there is a piece of small equipment that could be helpful for me to build abs.
If you're talking about reducing belly fat, you should read up that spot reduction is a myth: you can't target fat on any part of your body.
But directly answering your question the smallest piece of equipment I know of for abdominals is the ab wheel:
A caveat to the ab wheel is that it actually takes a pretty fit person to safely use it. If you can hold a plank for a minute or so, I think you're in a good spot. It can be a lot of load on wrists, shoulders, and your abs.
Used incorrectly, it can put a lot of load on your lower back as well.
Check gymnastics related workouts and you will be fine regarding core strength. No equipment required.
If you like to get new toys to keep you motivated and/or vary the exercises, paralettes can be a small and very useful piece of equipment, pull up bar and/or rings can also help
Most important thing in "buliding" your abs is low fat procentage - you proably even know some guys who have never done any exerciese but got some visible abs, just because of genetics.
So, exercieses are second, but matter as well.
So, if the question about an equipment I can recommend pull up bar chrunces as exercise where as an equimpent you can use just some dumbell - just hold it between your legs and perform crunches.