Learn Hardcore Icelandic: Þessi stærð passar henni virkilega vel.

Because with the meaning to fit (clothes/size fitting a person), the verb passa takes the dative case for the person. Henni is the dative singular of the 3rd‑person feminine pronoun. So you say passa e-m (fit someone, in the dative).

Useful dative pronouns:

me: mér
you (sg.): þér
him: honum
her: henni
it: því
us: okkur
you (pl.): ykkur
them: þeim

Using hana (accusative) or hún (nominative) would be ungrammatical here.

What does Þessi agree with, and why not þessa or þetta?

Þessi is the demonstrative determiner this, and it agrees with the noun in gender, number, and case. Stærð is feminine, nominative, singular as the subject, so you use þessi (fem./masc. nominative singular).

Use þessa when feminine accusative or genitive is needed.
Use þetta for neuter singular (nom./acc.), e.g., þetta stykki.

Why is stærð in the nominative case?

Because stærð is the subject of the sentence. The subject of a finite verb is in the nominative in Icelandic. The verb is passar (3rd person singular, present), agreeing with the subject þessi stærð. The person being fitted (henni) is in the dative as an object/experiencer.

Does passa always take the dative?

Only in the fit‑someone sense. Passa has several uses with different complements:

Fit (clothes/size) someone: passa e-m (dative). Example: Skórnir passa mér.
Match/go with: passa við e-ð (við + accusative). Example: Skórnir passa við kjólinn.
Look after/guard: passa e-n/e-ð (accusative). Example: Ég passa barnið.
Be careful: reflexive passa sig. Example: Passaðu þig!

Could I say Þessi stærð passar hana virkilega vel?

No. In the fit‑someone sense, passa requires the dative for the person, so it must be henni, not hana (accusative). The nominative hún is only for subjects.

Can the word order change, e.g., Henni passar þessi stærð virkilega vel?

Yes. Icelandic has verb‑second (V2) word order, so any constituent can be fronted for emphasis as long as the finite verb stays in second position:

Þessi stærð passar henni virkilega vel. (neutral)
Henni passar þessi stærð virkilega vel. (emphasis on her)
Virkilega vel passar henni þessi stærð. (heavy emphasis on how well; marked style)

Where do the adverbs go? Why virkilega vel after the verb?

Degree adverbs like virkilega typically modify another adverb or adjective. Here, virkilega intensifies vel, and the adverbial phrase virkilega vel naturally sits after the verb phrase:

Þessi stærð passar henni mjög/virkilega/sérstaklega vel. Placing virkilega before the verb (e.g., Þessi stærð virkilega passar...) is not idiomatic in neutral speech.

What’s the difference between passa, fara, and henta/hæfa?

passa e-m: fit physically (size/measurements). Example: Þessi stærð passar henni vel.
fara e-m: suit aesthetically/visually. Example: Þessi litur fer henni vel.
henta e-m: be convenient/appropriate for someone’s needs or schedule. Example: Þessi tími hentar mér.
hæfa e-m: befit/be appropriate to someone’s role/style; more formal. Example: Kjóllinn hæfir tilefninu.

How would I say this for other people (me/you/him/us/them)?

Keep the structure and change the dative pronoun:

Þessi stærð passar mér virkilega vel.
Þessi stærð passar þér virkilega vel.
Þessi stærð passar honum virkilega vel.
Þessi stærð passar okkur virkilega vel.
Þessi stærð passar ykkur virkilega vel.
Þessi stærð passar þeim virkilega vel.

How do I say it in the plural: These sizes fit her really well?

Use plural agreement on the determiner and noun; the verb is 3rd‑person plural:

Þessar stærðir passa henni virkilega vel. Here þessar = fem. nom. plural, stærðir = fem. nom. plural, and passa = 3rd plural present.

Can I add the definite suffix, like Þessi stærðin?

No. You don’t double‑mark definiteness in Icelandic. Use either a demonstrative or the suffixed article, not both:

With demonstrative: Þessi stærð passar henni...
With suffixed article (previously known item): Stærðin passar henni...

How do I put this in the past or express future?

Past (preterite): Þessi stærð passaði henni virkilega vel.
Future: Often the present is used for near future with context, but you can use mun: Þessi stærð mun passa henni virkilega vel.
Perfect: Þessi stærð hefur passað henni virkilega vel.

Any pronunciation tips for tricky sounds here?

Þ in Þessi: unvoiced dental fricative [θ], like thin.
æ in stærð: a diphthong [ai], like eye.
ð at the end of stærð: voiced dental fricative [ð]; the final cluster can sound like [stairð].
Double ss in passar: a long, unvoiced [sː].
nn in henni: long [nː].
In virkilega, the g between vowels is often a soft [ɣ].

What about passa við? Could I use that here?

No. Passa við means to match/go with something (style/color), and it governs the accusative with við:

Skórnir passa við kjólinn. (The shoes match the dress.) For size fitting a person, use plain passa
- dative person:
Þessi stærð passar henni (vel).

When would I use í þessari stærð instead?

Í þessari stærð means in this size and is common for availability or options:

Er þetta til í þessari stærð? (Is this available in this size?) Your sentence makes the size the subject and says that it fits her well: Þessi stærð passar henni virkilega vel.