Three overall lengths to check, and likely the last is only an issue under special conditions.  The case "headspace" length is more important as it determines function in the chamber.

The first is maximum magazine length; whether the loaded cartridge feed from the magazine
The second is whether sufficient bearing length is seated into the case neck for good neck tension.  Short lightweight bullets may not be sufficiently long to seat out to maximum magazine length.

The third is seating a bullet with a cannelure.  Seating to the cannelure sets the COAL.  However, there is no rule that seating to the cannelure is mandatory.

Last is the relation between the bullet ogive and the start of the rifling.  This is broken into three considerations, whether the bullet is jammed into the rifling and fired with loads that don't accommodate that condition (not for beginning reloaders), whether there is sufficient clearance for safety with handbook loads, and adjustment of "jump" to the rifling for tweaking accuracy (after learning the basics of safe and effective reloading).  This step requires a tool for measuring the length of the loaded cartridge from the case head to the bullet ogive.  Unless the rifle has a custom chamber specifically throated for a bullet, this won't generally be an issue with a SAAMI chamber if it was cut correctly.  Bad behavior of a rifle and ammo combination warrants a check of the chamber dimensions, especially the diameter of the chamber neck, the length of the neck, and the freebore.

As a new, green reloader you should jump over to the Armory Reloading forum for help to get up to speed.

I recommend at minimum the Hornady, Speer, and Lyman manuals (for cast bullets and handgun ammunition), including old versions as you find them used.  There are scads of cheap Speer manuals.

Each gunpowder seller prints a short manual of limited load data that can be useful, especially if a chamber pressure is included.

The front part of the LEE manual is excellent, but the load data needs to be compared to all the other data.

The Hornady reloading site has a section about the process that is the equal or better of others.

After you collect up your load data from different sources the first detail you will notice is that the maximum charge weights are different.  That is due to case, primer, bullet, powder lot, chamber details, and pressure measuring method.

It's not an obstacle.  Remember that published load data is a guideline, not gospel.  Remember that the advice to use start loads 10% below maximum is good practice.  (If the published start load is greater than that number, then use the published start load.)

Hornady data is currently measured with piezoelectric methods.  This allows finding the true (or close enough) pressure during the burn cycle.  That pressure is greater than those measured by the copper crusher method, and there is no useful accurate conversion between the two.  The crusher method uses cylinders (from a lot) mashed in a press to calibrate against the pressure in the press cylinder, then fired in a test weapon with a hole into the chamber where the crusher contacts the case wall.  The result is a measure of the work required to reflect the crusher, the change in length correlated to the calibration results.  The true maximum pressure is not measured.  Guns have not been blowing up by using this data.

Back to handling the discrepancies in the maximum charge weights -

Collect all of the maximum charge weights you can find.  This is the possible range of maximum charges.

Calculate the average of those maximums.  The average is the new, interim maximum charge.

Calculate the starting charge 10% down.  (Compare to the published minimums and any notes in the text; see the data for H110.)

Load and test your ammo and firearm.  Increase the charge weights in increments no smaller than 0.3 grains in .223 Rem cases or 0.5 grains in .308 Win cases, otherwise it's a waste of components.  I won't cover the search for the "best, most accurate load" , that's for a different thread.

Most likely you will be able to use charge weights as large as the maximum you found.  But eventually you'll find a rifle that can't swallow the minimum maximum charge you found, so caution and attention is always smart.

I modify the 10% down start load advice when I'm loading for a familiar rifle, cartridge, and either bullet or gunpowder by using start loads between 5% and 7% down from maximum to save components.  Which to choose depends on my experience with the components and whether I think extra caution is warranted.  Any new cartridge I have never loaded starts at 10% down from maximum charge weight.

I don't recommend drop in case gauges, especially Lyman gauges for .223 Rem cases as those gauges are not cut correctly and will cause rejected cases that are fine.  The other reason is that they cause the case shoulders to be set back unnecessarily short.

Look for bushing type gauges that allow a comparative measurement of the case headspace and cartridge base to bullet ogive length.  Accurate bushings from the parts bin are a great substitute if they are long enough.