In Brillouin sensing systems, single-mode fibers are commonly used. However, the input optical power of single-mode fibers is limited by the stimulated Brillouin scattering effect, which constrains their sensing performance. Compared with single-mode fibers, few-mode fibers have larger core radius, allowing higher input optical power and enabling the transmission of multiple relatively independent modes simultaneously, which holds the potential for improved sensing performance. Brillouin scattering in few-mode fibers can be characterized by parameters such as Brillouin gain peak, Brillouin frequency shift, and linewidth. These parameters vary with modes and exhibit different response characteristics to various physical quantities. Brillouin scattering in few-mode fibers offers the potential for simultaneous measurement of multiple physical quantities. Therefore, studying the Brillouin scattering characteristics of different modes in few-mode fibers is crucial, as it helps to better understand how each mode contributes to the overall scattering response and enables the optimization of the fiber's performance for multi-parameter sensing applications. However, existing research still has certain limitations. Reports on the full-mode acousto-optic mode coupling in few-mode fibers, as well as the effects of core radius and doping concentration on their stimulated Brillouin scattering, are scarce. To investigate the impact of core radius and doping concentration on the full-mode acousto-optic mode coupling of stimulated Brillouin scattering in few-mode fibers, this paper introduces the optical and acoustic waveguide characteristic equations of few-mode fibers under satisfying the core and cladding boundary conditions. Different optical and acoustic mode distributions and normalized field distributions are analyzed using characteristic functions and finite element methods. The relationship between the number of acoustic modes and the normalized acoustic frequency values in few-mode fibers is explored. The full-mode Brillouin gain spectrum of four-mode fibers is analyzed, and the effects of core radius and germanium doping concentration on the Brillouin frequency shift, linewidth, and gain spectrum in few-mode fibers are studied. The variation patterns of Brillouin frequency shift and linewidth in full-mode acousto-optic coupling of ten-mode fibers are summarized. In this paper, the simulations were implemented using MATLAB and COMSOL Multiphysics software. The research results indicate that there is a quadratic relationship between the number of acoustic modes in few-mode fibers and the acoustic normalized frequency value. The full-mode acousto-optic coupling in few-mode fibers can be divided into effective and ineffective acousto-optic mode coupling. Effective acousto-optic coupling can be divided into two cases: one is the coupling between the L_ln mode family (l=0) and any LP_mn mode, and the other is the coupling between the L_ln mode family (l=2m) and the LP_mn mode (m>0). When the germanium doping concentration is between 1.5% and 4.0%, the linewidth of the full-mode acousto-optic mode coupling increases with the increase of germanium doping concentration, and when the concentration exceeds 2.0%, the linewidth shows a linear increasing trend. Brillouin frequency shift variation: when the LP_mn mode families are coupled with the L_0n mode family, the Brillouin frequency shift decreases with increasing optical mode order; when the LP_mn mode families (m>0) are coupled with the L_ln mode families (l=2m), the Brillouin frequency shift approaches the total Brillouin gain spectrum frequency shift. Line width variation: when the LP_mn mode families (m=0) are coupled with the L_ln mode families (l=0), the line widths of the LP₀₂ and LP₀₃ modes are much larger than those of the LP₀₁ mode; when the LP_mn mode families (m>0) are coupled with the L_ln mode families (l=0) and the L_ln mode families (l=2m) respectively, and the linewidth of the same optical mode family LP_mn increases as the optical mode order increases when it is coupled with the acoustic mode. The research results of this paper are expected to provide certain reference value for stimulated Brillouin scattering in few-mode fibers.