If anything, you would likely want to size down if you plan to run drops. Frames designed for drop bars have shorter top tubes than frames designed for flat bars.
Of course, this also means that you’re likely going to need a few spacers or a riser stem, as the stack will likely be a bit too short.
Also, older mtbs (like pre-early 2000s) tend to be better for conversion since top tubes have gotten even longer since then.
Be aware that these conversions can be tricky, as there can be compatibility issues when mixing Road and MTB brakes/levers and shifter/FDs. There is a ton of information out there about this, so I’m not gonna go into details here.