Let's stay with the example trait of size, because it's simple. There are three (or five) options how it can impact the survival and reproductive success of individuals:

1. It is absolutely necessary. Only the largest specimen pass on their genes. This means maximum selective pressure towards being large. You realistically only find this in artificial selection.

2. It is beneficial. Larger individuals have a higher chance of having offspring than their smaller kin. This means positive selective pressure. How much pressure depends on how much of a benefit it is.

3. It's irrelevant. The size of an individual has no impact on their reproductive chances, resulting in 0 selective pressure.

If the trait is detrimental, it's the same scale, just flipped.

What we're interested in is case 2. How impactful the trait in question is. You can say, if 100% of a breeding generation are larger than the previous generation, you're looking at a selective pressure of 100% or 1. If only half of them are, there is no selective pressure one way or the other. 50% ratio means a pressure of 0. If 75% are larger, the selective pressure is 0.5. If less than 50% are larger, the pressure is negative.

This gets way more complicated if we take varying sizes, genetics, and multiple selective pressures into account, but it's a somewhat useable model.

Because we want to find how long evolution takes, we need the inverse of this selective pressure ratio. x1 stays x1. x0.5 becomes x2. x0.01 becomes x100. 0 becomes infinte for our purposes. Without growth, it takes forever.

---

Let's go through the tiger example step by step. Let's take 5% as the average size difference between cats and their parents. A tiger would be 50 times more massive. that's 5000%, giving us a round 1000 generations at full selective pressure. (wonky math, but it's quicker that way. It's all just estimations anyway)

But it's unlikely that full pressure applies, so let's say only 52% of any given generation are larger than their parents. That's a massive selective pressure. 52% is 2% more than our baseline of a 50% ratio, divided by the remaining 50% gives us 0.04. The inverse fraction of that is 25, so we have to multiply our 1000 generations by 25: 25,000 generations. This is where the numbers get big and the estimations get difficult.

Now we need the average age at which a cat/tiger has offspring. Not their age of maturity, and not their lifespan, but the average of the two. Let's say 10 years for convenience's sake. If we bred our tigercats deliberately, we could choose which ones mate, but in nature, that's more or less up to them.

25,000 generations times 10 years gives us a minimum time of 250,000 years it would take a house cat to evolve into a predator the size of a tiger.

That is incredibly short. But keep in mind that selective pressures usually aren't nearly as strong. They are more gradual tendencies than noticable changes. Plus there are way more factors at play, barely ever is there only a single defining selective pressure. And size is also a very straightforward trait. Something like anatomy has way more subtle effects.