I have never liked Fullsize boards honestly because unless your job requires it...honestly i don't see the point. Even should your job require it an external numpad is vastly superior IMO being able to move it where you like giving more space for your mousing arm. Myself i am fairly proficient with the number row anyways...however should i be doing a large amount of math i do have an external Kinesis Numpad next to my HHKB for when i do Taxes or formulate recipe costs etc. That said far as switching sizes i started with TKL size...found in my usage i never really used F keys or anything a TKL offers..so went down to 60% and been there ever since with multiple in my collection now. A TKL or larger just makes my desk feel cluttered now and i can't stand not having a 60% at this point. Even for gaming i have come to prefer 60% over TKL as the extra space and bringing your arms closer together while using a mouse at the same time is far more ergonomic (especially over a full size).
An external numpad would take up even more space. Besides, I use the numpad so often there's no point in me moving it around
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You have explained (quite well) why the numpad is essential for your job and of course those jobs get a bit of a pass. I doubt there is much you do that uses the mouse while typing at the same time. However for those of us that do the distance apart between your mousing hand and typing hand (Primarily gaming of course) is exceptionally not ergonomic and very uncomfortable. Also the difference in an external numpad is
A. I can put it away when i do not need it, which frankly is more often than not.
B. It gives me another place to put Artisans or spare caps

C. an External Numpad next to my HHKB is right on par the same size as my TKL KUL 87 board is and loads smaller than a Fullsize for sure.
D. Without modification it allows me to have different switches on my numpad than my keyboard. I highly prefer clickly tactile on the numpad for doing quick math where as for constant typing i prefer my HHKB or MX Clears as the noise of Clicky tactile can bother me over long periods of time.
Even were i in a position were constant access to a numpad was more or less a necessity such as your line of work i would still prefer a 60%+ ext numpad. Or if the Function row was also a requirement a TKL+ Ext numpad so i could move it to the left out of the way when not needed. In addition it just feels more "natural" to me on the left than on the right side.
It is of course exceptionally subjective and while i totally understand someone preferring a fullsize for myself it's just loads of uncomfortable and wasted space.
We aren't all chemists after all

The only time i need a numpad at work is when figuring out costs of new recipes or sending out orders online for bulk ingredients or figuring out profit % etc which generally is only 1-2 days a week. I would rather not have that extra space wasted the rest of the time by something i don't need at the moment.