What is Chrome default cache size limit?

There's isn't a UI to set the cache size limit, you'll have to do so using command line parameter

 --disk-cache-size=xxx (in bytes)

You can check the Current Cache size & Max size by heading over to chrome://net-internals/#httpCache page

Currently, Chrome ignores the values above 2GB ~ 2000000000

According to Chromium source, it uses kDefaultCacheSize = 80 * 1024 * 1024 Bytes --> 80MiB and applies the first matching rule of the following to determine the actual (disk, HTTP) cache size

• 80% of the available space if there is not enough space to use kDefaultCacheSize
• kDefaultCacheSize if it uses 10% to 80% of the available space
• 10% of the available space if the target size (2.5 * kDefaultCacheSize) is more than 10%
• the target size (2.5 * kDefaultCacheSize) if it uses 10% to 1% of the available space
• 1% of the available space

So that should be 10% of the available disk space at startup in the common case, with a moderately filled HDD.

There isn't a default size for Cache in Google Chrome.
The maximum size of the cache is calculated as a percentage of available disk space. The default size is calculated at start time.

From: http://gent.ilcore.com/2011/02/chromes-10-caches.html

The maximum size of the cache is calculated as a percentage of available disk space. The contents can be viewed at chrome://net-internals/#httpCache. It can be cleared manually at chrome://settings/advanced or programmatically by calling chrome.benchmarking.clearCache() when Chrome is run with the --enable-benchmarking flag set. Note that for incognito windows this cache actually resides in memory.

Also, the article explains other 10 type of Caches like Memory Cache, DNS Cache etc.