Please explain usage of " ATLEAST "in degrees of comparision.

You are comparing things with each other so comparative degree.

"John is at least as big as Jim."

That is comparing two objects.
You did not say:
"John is big." (positive)
You did not say:
"John is the biggest of all."(superlative)

"In the context of degrees of comparison, the term 'at least' is not used as a specific degree, but rather as an expression denoting a minimum or lower limit. It is typically used to indicate that a certain quality or quantity is the minimum or baseline level, and it can be used with both positive and negative forms of comparison."

To clarify, let's consider the usage of "at least" in different degrees of comparison:

1. Positive degree: The positive degree is used to simply describe a quality or quantity without any comparison. When using "at least" in the positive degree, it implies that the described quality or quantity is the minimum or baseline level. For example:
- "He is at least 6 feet tall." (indicates that his height is a minimum of 6 feet)

2. Comparative degree: The comparative degree is used to compare two items or entities, typically using "-er" or "more" to indicate a higher degree. When using "at least" in the comparative degree, it suggests that the described quality or quantity is higher than what is being compared, but still at a minimum level. For example:
- "She is at least taller than her brother." (implies that she is taller than her brother, but the exact difference is not specified)

3. Superlative degree: The superlative degree is used to compare three or more items or entities, typically using "-est" or "most" to indicate the highest degree. When using "at least" in the superlative degree, it signifies that the described quality or quantity is the minimum among the group being compared. For example:
- "This is the book that I enjoyed at least the most." (suggests that among the books I enjoyed, this book was the minimum level of enjoyment, although it may not be the absolute favorite)

In summary, "at least" is a phrase used in degrees of comparison to denote a minimum or lower limit of a quality or quantity, regardless of whether it is in the positive, comparative, or superlative degree.