Backport PR #25584 on branch 0.24.x (BUG: Redefine IndexOpsMixin.size, fix #25580)
Manual backport of https://github.com/pandas-dev/pandas/pull/25584
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25667?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`0.24.x@fd4bc76`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25667/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25667?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             0.24.x   #25667   +/-   ##
=========================================
  Coverage          ?   92.39%           
=========================================
  Files             ?      166           
  Lines             ?    52424           
  Branches          ?        0           
=========================================
  Hits              ?    48437           
  Misses            ?     3987           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.83% <100%> (?)` | |
| #single | `42.9% <100%> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25667?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/25667/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYmFzZS5weQ==) | `97.75% <100%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25667?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25667?src=pr&el=footer). Last update [fd4bc76...84bb457](https://codecov.io/gh/pandas-dev/pandas/pull/25667?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25667?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`0.24.x@fd4bc76`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25667/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25667?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             0.24.x   #25667   +/-   ##
=========================================
  Coverage          ?   92.39%           
=========================================
  Files             ?      166           
  Lines             ?    52424           
  Branches          ?        0           
=========================================
  Hits              ?    48437           
  Misses            ?     3987           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.83% <100%> (?)` | |
| #single | `42.9% <100%> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25667?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/25667/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYmFzZS5weQ==) | `97.75% <100%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25667?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25667?src=pr&el=footer). Last update [fd4bc76...84bb457](https://codecov.io/gh/pandas-dev/pandas/pull/25667?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
