The translators all seems to miss the sense of this passage which is odd because Paul is speaking very plainly. The problem seems to be that translators tend to jump to the idea of μέτρον being a "portion" as if believers each get a slice of pizza. Paul is not speaking of each having a "portion" of faith but rather of each having a "measuring device" of faith as people are wont to compare their slice to the other slices rather than valuing and appreciating the value of their special slice.:
Westcott and Hort / [NA27 variants] Romans 12:3 Λέγω γὰρ διὰ τῆς
χάριτος τῆς δοθείσης μοι παντὶ τῷ ὄντι ἐν ὑμῖν μὴ ὑπερφρονεῖν παρ' ὃ
δεῖ φρονεῖν, ἀλλὰ φρονεῖν εἰς τὸ σωφρονεῖν, ἑκάστῳ ὡς ὁ θεὸς
ἐμέρισεν μέτρον πίστεως.
μέτρον, ου, τό (Hom.+; ins, pap, LXX, En, TestSol 15:5; TestAbr A; Test12Patr; GrBar 6:7; ApcMos 13; Sib Or 3, 237; EpArist, Philo; Jos., Ant. 13, 294, C. Ap. 2, 216; Just., 112, 4; Tat. 27, 3; Ath.) gener. **‘that by which anything is measured’.
① an instrument for measuring, measure**
ⓐ of measures of capacity ἐν μέτρῳ μετρεῖν Mt 7:2; Mk 4:24; 1 Cl 13:2b. μέτρῳ μετρεῖν (Maximus Tyr. 32, 9c; 35, 2i) Lk 6:38b; 1 Cl 13:2a; Pol 2:3. W. heaping up of attributes μ. καλὸν πεπιεσμένον σεσαλευμένον ὑπερεκχυννόμενον good measure, pressed down, shaken together, running over Lk 6:38a. In imagery: πληροῦν τὸ μ. τινός fill up a measure that someone else has partly filled Mt 23:32.
ⓑ of linear measure Rv 21:15. μέτρον ἀνθρώπου, ὅ ἐστιν ἀγγέλου a human measure, used also by angels vs. 17.
② the result of measuring, quantity, number
ⓐ lit. τὰ μ. τῶν τῆς ἡμέρας δρόμων φυλάσσειν keep the measure of its daily courses Dg 7:2.
ⓑ fig. (Maximus Tyr. 40, 3c ὑγείας μ.; Alex. Aphr., Quaest. 3, 12 II/2 p. 102, 2 μ. τῆς ἀληθείας; Ath. 32, 2 δικαιοσύνης μ.; 33, 1 μ. ἐπιθυμίας ἡ παιδοποιία) ὡς ὁ θεὸς ἐμέρισεν μέτρον πίστεως as God has apportioned the measure of faith Ro 12:3 (CCranfield, NTS 8, ’62, 345–51: Christ is the measure of faith). ἑνὶ ἑκάστῳ ἐδόθη ἡ χάρις κατὰ τὸ μ. τῆς δωρεᾶς τοῦ Χριστοῦ grace was given to each one according to the measure (of it) that Christ gave Eph 4:7. κατὰ τὸ μ. τοῦ κανόνος οὗ ἐμέρισεν ἡμῖν ὁ θεὸς μέτρου according to the measure of the limit (=within the limits) which God has apportioned us (as a measure) (s. B-D-F §294, 5; Rob. 719) 2 Cor 10:13. κατʼ ἐνέργειαν ἐν μέτρῳ ἑνὸς ἑκάστου μέρους according to the functioning capacity of each individual part Eph 4:16 (ἐν μ. as Synes., Ep. 12 p. 171c). καταντᾶν εἰς μ. ἡλικίας τοῦ πληρώματος τοῦ Χριστοῦ attain to the measure of mature age (or stature of the fullness) of Christ vs. 13 (s. ἡλικία 2a and cp. μ. ἡλικίας Plut., Mor. 113d; μ. ἥβης Il. 11, 225; Od. 11, 317).—οὐκ ἐκ μέτρου J 3:34, an expr. not found elsewh. in the Gk. language, must mean in its context not from a measure, without (using a) measure (the opp. is ἐν μέτρῳ Ezk 4:11, 16; Jdth 7:21).—DELG. M-M. TW. Sv.
Arndt, W., Danker, F. W., Bauer, W., & Gingrich, F. W. (2000). A Greek-English lexicon of the New Testament and other early Christian literature (3rd ed., p. 644). Chicago: University of Chicago Press.
He's saying, for example, if your gift is teaching, don't measure your teaching gift by comparing it to others who lack the gift and thus imagine (overestimate) yourself better than others. Rather, measure by how accurate you are. If giving, measure by how free your giving is from "strings". And so on.
He speaks of the same subject here:
2Co 10:12 For we dare not make ourselves of the number, or compare
ourselves with some that commend themselves: but they measuring
themselves by themselves, and comparing themselves among themselves,
are not wise. 2Co 10:13 But we will not boast of things without our
measure, but according to the measure of the rule which God hath
distributed to us, a measure to reach even unto you. 2Co 10:14 For
we stretch not ourselves beyond our measure, as though we reached not
unto you: for we are come as far as to you also in preaching the
gospel of Christ: 2Co 10:15 Not boasting of things without our
measure, that is, of other men's labours; but having hope, when your
faith is increased, that we shall be enlarged by you according to our
rule abundantly, 2Co 10:16 To preach the gospel in the regions
beyond you, and not to boast in another man's line of things made
ready to our hand. 2Co 10:17 But he that glorieth, let him glory in
the Lord. 2Co 10:18 For not he that commendeth himself is approved,
but whom the Lord commendeth.
KJV unless otherwise noted.