How big is the Holi festival in India?

The answer to your question will heavily depend on which region of India you are planning to visit. Holi is more prevalent in North India. In south Indian states like Tamil Nadu and Kerala for example, the color aspect of Holi is celebrated only in a few areas in the cities, typically where either North Indians have settled or where more young people live. It is also much more contained here.

The colors are usually bought from stores/roadside vendors by individuals or even groups organizing it in their local community. Be warned though, not all colors are safely made and some might cause chemical burn/rashes to those with sensitive skin. So if you do plan to celebrate, you might want to take the precaution of checking the source of the colors and dyes.

It is tremendous fun, though it can turn rowdy at times and you need to be careful of your possesions. If you only want to experience the festival, depending on where country you live in, the local Indian community might organize Holi or you could participate in events in the US such as Color Run, which is essentially a mix of Holi and a 5k run.

Holi typically takes place in the months of March-April (the exact dates vary from year-to-year since it's based on the Hindu lunar calendar). And indeed, it is a massive festival...in the Northern regions of India, since that's where the tradition is based. It is also celebrated in other parts of India in slightly different forms, but not the same extent.

Holi doesn't just involve playing with powdered colours, but also water fights with water pistols and water balloons. Suffice to say that if you're out on the streets during the festival, you will get soaked and caked in colours. The festivities drag on over multiple days. Another tradition during Holi, which you may come across when travelling around this time, is the consumption of bhang - an extract from cannabis that is typically crushed, mixed with water, and drunk to get intoxicated. (Recreational drugs are illegal in India but on this occasion it's pretty much overlooked because of "tradition").

(As an aside, the Thai festival of Songkran takes its roots from the Indian tradition of Holi, and is celebrated in somewhat similar ways with water fights.)

The powdered colours themselves can be bought from shops, or people will be more than willing to share with you as long as you agree to join them in celebrating. Concerns have been raised over the past few years on the safety of the dyes used in some of these colours as the cheaper ones tend to be made using metallic oxides. Dark blue and black colours tend to be of this kind. If you want safer dyes, ask for gulal (pronounced "goo-la-ahl") which are made from natural plant extracts and usually won't cause allergic reactions. Limited contact to the cheaper metallic dyes shouldn't cause problems for most people either; it's only long term exposure that can cause damage to skin, but it's something worth being aware about.

Indian expatriates around the world celebrate Holi too, so in case you're unable to travel, it's possible to get a taste of the festival at one of these celebrations. Check out this video at Festival of Colour USA - they can be pretty large scale events too!