Is it a good practice to use the abbreviation resp. for respectively in scientific writing ? Let consider the following sentence as example. "The word size (resp., word length) is defined as the number of different values (resp., significant digits) that one word can store (resp., align)."
It's not about the abbreviation. Your use of the word respectively is wrong.
'Respectively' is used to say that items in a list correspond with each other in order. For example:
Alice and Bob live in apartments 1 and 2, respectively.
This means Alice lives in apartment 1 and Bob lives in apartment 2. Without the word 'respectively' we know only that both Alice and Bob live in either apartment 1 or apartment 2.
A useful word that people sometimes mean when using 'respectively' is 'specifically'. It means we qualify a general term with a specific one. For example:
If you sort a list of countries by size (specifically land area) Russia is at the top.
The vague word 'size' is explained to mean the more specific term 'land area', rather than other possibilities such as land and sea area, or population.
However none of the examples you quote are really appropriate for either 'respectively' or 'specifically'
- 'word length' is not a more specific term for 'word size', it's just another name for the same thing.
- The 'number of different values' is not the same as the 'number of significant digits. The 'number of different values' of a byte is 256, whereas the number of significant digits is 8 (binary) or 3 (decimal). The word length of a byte is of course 8.
- The word 'align' does not appear to have any relationship to the word 'store'.
The word size (also called word length) is defined as the number of significant digits (in the appropriate base) that a word can store.
The examples and the abbreviation appear to reflect the way the conjunction "respektiv(e)" is used, and abbreviated, in German. So no, it is not good practice to use it in English, as it may mystify non-German-speaking readers. As noted above, this is not at all how "respectively" is used in English.
The correct use is modelled in the "Alice and Bob" example above. German "respektive" means "or" or (not resp.!) "or else/rather". It's a mistake German L1 users of English often, and understandably, make. (This observation is based on 35 years of teaching academic English in Germany and on the Cambridge defintions of respektive and respectively.