Now

That which is neither the past nor the future. Now is equal to plancks time.

Planck Time : t_p = 5.39 × 10^-44 s

Before we could get to present, lets understand past and future.

Past is the one we experience through our sense organs. What we see through our eyes or hear through our ears are events that happened some time in the past. light and sound travels with some fixed velocity. we may say all our physical experiences are from the past.

we may at times see the past in our dreams. so past is also from our memories (which in-turn are from our experiences).

Future happens in our mind. it is the one that is projected from our past experiences and experiences are gained through our sense organs.

Now. Now is neither past nor future (similar to absence of light being darkness).

Dictionary Definitions of Now: That which is immediate or in the present time.

So, what could the words Immediate or Present mean?

Philosophical Perspective (From Swami Vivekananda): Present is the duration between an inhalation and exhalation. There is no past or future ( when our body looses the ability to breath).

Physiological Perspective: That's approximately linked to the rate of breathing. Could be 10 secs for a human being at rest (it can increase or decrease depending on the activity we are involved in)

We can go on... Now is the time between our birth and death, live one day at a time... it goes on...

So, how small is now. Scientifically, it could be equal to plancks constant.  Its could be in terms of energy (plancks constant) or space (plancks length) or time (plancks time) or mass (plancks mass). 

Spiritual Perspective: The state of being in which there are no sensory inputs or mental thoughts. The state of meditation. the state of stillness.  the state of experiencing  plancks time. the eternal state of bliss or being one with atman.

In being one with atman may be we can travel through this universe back and forth in time effortlessly or getting to understand that, being in this physical form, absolute now is an impossibility. This could be meaning of awareness.

May be we can say NOW depends on form, sensory inputs and state of mind (human, tree, bacteria, muon etc).

If we have to be in the NOW, we got to be space or time or mass or energy. Everything is derived from us.

Ha ha. That's more like saying "You are God"

Note:

Sensitivity of our sense organs may also be considered as another perspective towards "Now". Eyes have approximately 15ms sensitivity (24 frames per second for a movie). Ears have 30 ms sensitivity (reason why we hear echo). Touch should also be within 30ms limit (experiment by moving a finger across a candle flame).  Taste and smell should also be within the same range of 15 to 30 ms (not sure of how to conduct an experiment for this).

So "Now" could be a time between 15 to 30ms range from the perspective of sense organs.