With the charts below and knowing or deducing the nut / bolt thread size, you can figure what size wrench is needed.
If you are not sure of the thread size, you can reference the Hardware P/N & Descriptions pages in the Sportsterpedia to find bolt sizes for IH and Evo Sportsters.
And likewise, if you know the wrench size, you can sometimes deduce the thread size (barring course or fine threading).

Let's say you have a bolt / nut you need to remove but you don't know what size wrench to use.
First and foremost, use the appropriate wrench type (6 point hex requires a 6 point wrench).
Using a 12 point wrench on a 6 point hex will eventually result in either rounding the hex corners or stripping / breaking the wrench.
Likewise, if a 6 point wrench even fits on a 12 point nut / bolt, it will not be able to keep enough torque on the corners and will end up stripping them.

You can measure across two opposite flats of a hex bolt / nut with a tape measure or yard stick. But it will not be as accurate as using a caliper.
Either way, get the measurement and choose a wrench that falls in the nearest size up from the hex size.
Wrenches come in standard sizes and most, but not all, hex bolts / nuts are made with standard wrench sizes in mind.
The real trouble comes when companies use “special manufacturing Specs” (most likely so you'll have to buy their tool to fit the part).

In the pic below, an engine sprocket nut from a Buell XB crank was measured at .806“ (20.47mm). The next size up SAE wrench size is .8125” or 13/16“ (20.6375mm).

¹⁾

When you reference the hex wrench chart below, be aware that using a larger size wrench will increase the chances of stripping the bolt head corners. In some cases, you may want to try using a size above the one recommended in the chart to see if it fits by chance. Not every metric size has a perfect SAE counterpart and vice-versa. There is no perfect substitute for the right size tool.

SAE Hex Wrench Size	5/16”	3/8“	7/16”	1/2“	9/16”	5/8“	3/4”	13/16“	7/8”	15/16“
SAE Bolt Size	1/8”	3/16“	1/4”	5/16“	3/8”	7/16“	1/2”	9/16“	9/16”	5/8“
Metric Hex Wrench Size	8mm	10mm	11mm	13mm	14mm	16mm	19mm	21mm	22mm	24mm

When you reference the hex key chart below, be aware that using a smaller size bit will increase the chances of stripping the corners. ²⁾ In some cases, you may want to try using a size above the one recommended in the chart to see if it fits by chance. Not every metric size has a perfect SAE counterpart and vice-versa. There is no perfect substitute for the right size tool.

SAE Hex Key	5/64”	3/32“	7/64”	1/8“	5/32”	3/16“	7/32”	1/4“	5/16”	3/8“
Metric Hex Key	2mm	2-1/2mm	3mm	3-1/2mm	4mm	5mm	6mm	7mm	8mm	10mm